Improve error reporting in protoc-gen-go-googlecetypes

[grayside]

Current Behavior

Each function in the generator uses log.Fatal or panic on encountering an error.

Improved Behavior

Return an error from the function and centralize error handling.

Preferred Behavior

Aggregate all errors from the parsing process and report at once. This has two facets:

1. When assembling data for use in code generation, assemble in advance, aggregate all errors, then flush at the end.
2. Instead of halting the generate-code.sh script on any error, continue processing but ensure to fail the script if any event type has failures