fix: Report proc macro errors in expressions correctly as well

They didn't have a krate before, resulting in the generic "proc macro
not found" error.

Also improve error messages a bit more.

crates/hir-expand/src/proc_macro.rs

@@ -75,7 +75,7 @@ impl ProcMacroExpander {
                     },
                 }
             }
-            None => ExpandResult::only_err(ExpandError::UnresolvedProcMacro),
+            None => ExpandResult::only_err(ExpandError::UnresolvedProcMacro(self.krate)),
         }
     }
 }