v5.4.1: Fix import statement generation bug

This release includes two bug fixes.

First, there was a bug in import statement generation when multiple .proto files were imported which shared an identical protobuf package, but which had different output go packages. This would most commonly occur when importing multiple "well-known type" definitions, like google/protobuf/empty.proto and google/protobuf/wrappers.proto. This was reported in #104, and fixed in #107.

Second, generated Go clients would previously return non-nil response types even when returning non-nil errors. This led to confusing behavior in some obscure cases, as users usually expect non-nil error to imply a nil response. This was fixed in #105.