fix: return scan error if exists in parseDiff func

[chasay]

Description

When running Bearer with the --diff option and the scanner encounters an error it will return a Error: signal: broken pipe. This is because the error is not checked before returning from the parseDiff function.

Example command: go run ./cmd/bearer/main.go scan ~/code/repo-with-long-line-diff --diff

An example error would be a diff that contains a line longer than the MaxScanTokenSize. (64 * 1024 bytes). With this additional code, the returned error will be Error: bufio.Scanner: token too long.

I'll create a Github issue to accommodate a more thorough fix for handling long lines of diff.

Checklist

• I've added test coverage that shows my fix or feature works as expected.
• I've updated or added documentation if required.
• I've included usage information in the description if CLI behavior was updated or added.
• PR title follows Conventional Commits format

[gotbadger]

closed in favour of #1512