Report when error declaration in signature is needed

While looking at the assignments in the `defer`, if we encounter an
`error`-type assignment, emit a diagnostic if the error named in the lhs
of the assignment is not present in the return signature's names.

Not sure if this makes sense to me yet. I probably only want to report a
problem for a non-declare assignment of an error value when the error
type in the return signature is _not_ named at all.

1 files changed, 1 insertions, 1 deletions

diff --git a/testdata/signature.go b/testdata/signature.go
index 6fc15fc..d780240 100644
--- a/testdata/signature.go
+++ b/testdata/signature.go
@@ -2,7 +2,7 @@ package main
 
 import "errors"
 
-func shouldDeclareErrInSignature() error { // want "return signature should be '(err error)'"
+func shouldDeclareErrInSignature() error { // want "return signature should be '\\(err error\\)'"
 	var err error // Should use variable declared in signature
 
 	err = nil