chore: improve error message

see #346

src/Lean/Elab/App.lean

@@ -553,7 +553,7 @@ private def resolveLValAux (e : Expr) (eType : Expr) (lval : LVal) : TermElabM L
       | some (baseStructName, fullName) => pure $ LValResolution.const baseStructName structName fullName
       | none   =>
         throwLValError e eType
-          m!"invalid field notation, '{fieldName}' is not a valid \"field\" because the environment does not contain '{Name.mkStr structName fieldName}'"
+          m!"invalid field '{fieldName}', the environment does not contain '{Name.mkStr structName fieldName}'"
     -- search local context first, then environment
     let searchCtx : Unit → TermElabM LValResolution := fun _ => do
       let fullName := Name.mkStr structName fieldName

tests/lean/346.lean

@@ -8,3 +8,6 @@ def a : SomeType := SomeType.x
 end SomeType
 
 #eval SomeType.b
+
+def f (x : Nat) :=
+  x.z

tests/lean/346.lean.expected.out

@@ -1,3 +1,7 @@
 346.lean:10:6-10:16: error: unknown constant 'SomeType.b'
 346.lean:10:0-10:16: error: failed to synthesize
   Lean.Eval ?m
+346.lean:13:2-13:5: error: invalid field 'z', the environment does not contain 'Nat.z'
+  x
+has type
+  Nat