🐛️ Handle unimported modules properly

helpful error, not panic!

Closes #2422

compiler/problem/src/can.rs

@@ -161,10 +161,37 @@ pub enum RuntimeError {
         region: Region,
         exposed_values: Vec<Lowercase>,
     },
+    /// A module was referenced, but hasn't been imported anywhere in the program
+    ///
+    /// An example would be:
+    /// ```roc
+    /// app "hello"
+    ///     packages { pf: "platform" }
+    ///     imports [ pf.Stdout]
+    ///     provides [ main ] to pf
+    ///
+    /// main : Task.Task {} [] // Task isn't imported!
+    /// main = Stdout.line "I'm a Roc application!"
+    /// ```
     ModuleNotImported {
+        /// The name of the module that was referenced
         module_name: ModuleName,
+        /// A list of modules which *have* been imported
         imported_modules: MutSet<Box<str>>,
+        /// Where the problem occured
         region: Region,
+        /// Whether or not the module exists at all
+        ///
+        /// This is used to suggest that the user import the module, as opposed to fix a
+        /// typo in the spelling.  For example, if the user typed `Task`, and the platform
+        /// exposes a `Task` module that hasn't been imported, we can sugguest that they
+        /// add the import statement.
+        ///
+        /// On the other hand, if the user typed `Tesk`, they might want to check their
+        /// spelling.
+        ///
+        /// If unsure, this should be set to `false`
+        module_exists: bool,
     },
     InvalidPrecedence(PrecedenceProblem, Region),
     MalformedIdentifier(Box<str>, roc_parse::ident::BadIdent, Region),