chore: preserve and log full error backtraces in generate-updates

[torreypayne]

Part of stack to address b/542740217 (Apiary library generation errors preventing releases)

See bug for artifacts showing validation of the error output

Summary

When toys generate-updates encounters an error generating a gem (e.g. bin/generate-api exits non-zero), the exit code and error backtraces were not captured or printed in the final summary output.

Fix

1. Updated regen_single_gem and clean_old_gems in .toys/generate-updates.rb to capture stdout and stderr stream output via out: [:capture, :inherit], err: [:capture, :inherit].
2. When generation fails, the error details and exit code are logged immediately and stored in @errors.
3. Updated the end-of-run summary in generate-updates.rb to print full error messages and backtraces for all failed gems.
4. Passed through --verbose to bin/generate-api if Toys verbosity is enabled (verbosity > 0).

[aandreassa]

Please change it to chore since it doesn't affect clients.

Make sure to double check the commit message before you merge too (since it was originally pushed as a fix commit and that differs from the title).