Improve error reporting in protoc-gen-go-googlecetypes

## 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

Start by tracing the protoc-gen-go-googlecetypes generator functions that currently call log.Fatal or panic, then inspect generate-code.sh. Determine how parsing and event-type generation failures are propagated and reported; done means errors are aggregated as described and the script still fails when any event type has failures.

Written by the indexing model from the issue text.