Handle failure from grep/awk

[jsoref]

• fixes #255

[jsoref]

If the [ -z "$expected_checksum" ] condition was reached, then echo "Failed to extract checksum for ${platform_filename}" would have run, and the log would have had Failed to extract checksum for , but that isn't present. Thus that line can't possibly have been reached.

503s from github happen often enough, especially at the scale that your code runs.

The || true should cause your code to fall into the -z "..." case which is desirable.