CI/CD: a failed job's reason names its first error

The reason shown for a failed job, in the console and on its GitHub check run, was the last line of its log, which for a build is often a line such as error: build failed, waiting for other jobs to finish.... It is now the first error line that names a cause (for example error[E0308]: mismatched types), or a No space left on device line when the machine ran out of disk. When a long log is shortened, every error line is kept, including error: lines without a code, and a compiler's quoted source lines are no longer taken for an error.