Invalid Argument errors provide per field details. (#110)

* Invalid Argument errors now return a per field error message.

* Creating an API error struct.

* SDK uses field error details proto.

* Adding tests specifically for handler helper error messages resulting in expected API output.

* Inlining the error checks into the table driven tests.  Use protocmp for assertions to make printing out diffs easier to parse.

* Aggregate the invalid argument fields detected to be wrong.

* Make the returned order of fields deterministic in the error details..

* Fixing formatting issue in errors_test.go.

* Test to confirm unimplemented results in a 405 and not 404.

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/golang-migrate/migrate/v4 v4.11.0
 	github.com/golang-sql/civil v0.0.0-20190719163853-cb61b32ac6fe
 	github.com/golang/protobuf v1.4.2
+	github.com/google/go-cmp v0.4.0
 	github.com/grpc-ecosystem/grpc-gateway v1.14.6
 	github.com/hashicorp/dbassert v0.0.0-20200602132714-30dfb78f0133
 	github.com/hashicorp/errwrap v1.0.0