doc: elabissue for two improvable error messages

tests/elabissues/anonymous_constructor_error_msg.lean

@@ -0,0 +1,22 @@
+/-
+This is an annoyingly unhelpful error message that I see all the time.
+An exact line number would help a lot, but even so it would be great
+to replace the `⟨...⟩` with the actual term in question.
+-/
+structure Foo := (n : Nat)
+
+def Foo.sum (xs : List Foo) : Foo :=
+xs.foldl (λ s x => ⟨s.n + x.n⟩) ⟨0⟩
+
+#check
+let x1 := ⟨1⟩;
+let x2 := ⟨2⟩;
+let x3 := ⟨3⟩;
+let x4 := ⟨4⟩;
+let x5 := ⟨5⟩;
+let x6 := ⟨6⟩;
+Foo.sum [x1, x2, x3, x5, x6]
+/-
+error: invalid constructor ⟨...⟩, expected type is not an inductive type
+  ?m_1
+-/

tests/elabissues/invalid_field_notation_error_msg.lean

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+/-
+This is an annoyingly unhelpful error message that I see frequently.
+It reports the prefix of the field notation, but not the field that we are trying to access.
+An exact line number would help, but even so it would be great to report the field as well.
+-/
+structure Foo := (n : Nat)
+def Foo.f1 (f : Foo) : Nat := f.n
+def Foo.f2 (f : Foo) : Nat := f.n
+def Foo.f3 (f : Foo) : Nat := f.n
+def Foo.f4 (f : Foo) : Nat := f.n
+def Foo.f5 (f : Foo) : Nat := f.n
+
+#check (λ f g h =>
+  let x : Foo := ⟨f.n + 1⟩;
+  let y : Foo := ⟨g.n + 1⟩;
+  (λ f g h => f.f1 + g.f2 + h.f3 + f.f4 + g.f5 + h.f6) f g h)
+/-
+/home/dselsam/omega/lean4/tests/elabissues/invalid_field_notation_error.lean:8:0: error: invalid field notation, type is not of the form (C ...) where C is a constant
+-/