Turn invalid record field types into runtime errors

By emitting a runtime error rather than panicking when we can't layout
a record, we help programs like

```
main =
    get = \{a} -> a
    get {b: "hello world"}
```

execute as

```
Mismatch in compiler/unify/src/unify.rs Line 1071 Column 13
Trying to unify two flat types that are incompatible: EmptyRecord ~ { 'a' : Demanded(122), }<130>

🔨 Rebuilding host...
── TYPE MISMATCH ───────────────────────────────────────────────────────────────

The 1st argument to get is not what I expect:

8│      get {b: "hello world"}
            ^^^^^^^^^^^^^^^^^^

This argument is a record of type:

    { b : Str }

But get needs the 1st argument to be:

    { a : a }b

Tip: Seems like a record field typo. Maybe a should be b?

Tip: Can more type annotations be added? Type annotations always help
me give more specific messages, and I think they could help a lot in
this case

────────────────────────────────────────────────────────────────────────────────

'+fast-variable-shuffle' is not a recognized feature for this target (ignoring feature)
'+fast-variable-shuffle' is not a recognized feature for this target (ignoring feature)
Done!
Application crashed with message

    Can't create record with improper layout

Shutting down
```

rather than the hanging

```
Mismatch in compiler/unify/src/unify.rs Line 1071 Column 13
Trying to unify two flat types that are incompatible: EmptyRecord ~ { 'a' : Demanded(122), }<130>

thread '<unnamed>' panicked at 'invalid layout from var: UnresolvedTypeVar(104)', compiler/mono/s
rc/layout.rs:1510:52
```

that was previously produced.

Part of #2227

compiler/types/src/pretty_print.rs

@@ -519,7 +519,8 @@ fn write_flat_type(env: &Env, flat_type: &FlatType, subs: &Subs, buf: &mut Strin
             let RecordStructure {
                 fields: sorted_fields,
                 ext,
-            } = gather_fields(subs, *fields, *ext_var);
+            } = gather_fields(subs, *fields, *ext_var)
+                .expect("Something ended up weird in this record type");
             let ext_var = ext;
 
             if fields.is_empty() {

compiler/types/src/subs.rs

@@ -1968,7 +1968,8 @@ impl RecordFields {
         subs: &'a Subs,
         ext: Variable,
     ) -> impl Iterator<Item = (&Lowercase, RecordField<Variable>)> + 'a {
-        let (it, _) = crate::types::gather_fields_unsorted_iter(subs, *self, ext);
+        let (it, _) = crate::types::gather_fields_unsorted_iter(subs, *self, ext)
+            .expect("Something weird ended up in a record type");
 
         it
     }
@@ -2003,7 +2004,8 @@ impl RecordFields {
                 ext,
             )
         } else {
-            let record_structure = crate::types::gather_fields(subs, *self, ext);
+            let record_structure = crate::types::gather_fields(subs, *self, ext)
+                .expect("Something ended up weird in this record type");
 
             (
                 Box::new(record_structure.fields.into_iter()),

compiler/types/src/types.rs

@@ -1701,14 +1701,20 @@ pub fn name_type_var(letters_used: u32, taken: &mut MutSet<Lowercase>) -> (Lower
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+pub struct RecordFieldsError;
+
 pub fn gather_fields_unsorted_iter(
     subs: &Subs,
     other_fields: RecordFields,
     mut var: Variable,
-) -> (
-    impl Iterator<Item = (&Lowercase, RecordField<Variable>)> + '_,
-    Variable,
-) {
+) -> Result<
+    (
+        impl Iterator<Item = (&Lowercase, RecordField<Variable>)> + '_,
+        Variable,
+    ),
+    RecordFieldsError,
+> {
     use crate::subs::Content::*;
     use crate::subs::FlatType::*;
 
@@ -1733,7 +1739,7 @@ pub fn gather_fields_unsorted_iter(
             // TODO investigate apparently this one pops up in the reporting tests!
             RigidVar(_) => break,
 
-            other => unreachable!("something weird ended up in a record type: {:?}", other),
+            _ => return Err(RecordFieldsError),
         }
     }
 
@@ -1749,11 +1755,15 @@ pub fn gather_fields_unsorted_iter(
             (field_name, record_field)
         });
 
-    (it, var)
+    Ok((it, var))
 }
 
-pub fn gather_fields(subs: &Subs, other_fields: RecordFields, var: Variable) -> RecordStructure {
-    let (it, ext) = gather_fields_unsorted_iter(subs, other_fields, var);
+pub fn gather_fields(
+    subs: &Subs,
+    other_fields: RecordFields,
+    var: Variable,
+) -> Result<RecordStructure, RecordFieldsError> {
+    let (it, ext) = gather_fields_unsorted_iter(subs, other_fields, var)?;
 
     let mut result: Vec<_> = it
         .map(|(ref_label, field)| (ref_label.clone(), field))
@@ -1761,10 +1771,10 @@ pub fn gather_fields(subs: &Subs, other_fields: RecordFields, var: Variable) ->
 
     result.sort_by(|(a, _), (b, _)| a.cmp(b));
 
-    RecordStructure {
+    Ok(RecordStructure {
         fields: result,
         ext,
-    }
+    })
 }
 
 pub fn gather_tags_unsorted_iter(