Fix parsing of diagnostics from binaries

[Jake-Shadle]

When spirv tools output error messages, particularly validation, they often print out a block of extra information (typically instructions) after the top level error message. The compiled versions were "handling" this since it was all stuffed into a single string, but when parsing the messages from stderr we were just discarding this additional information since they weren't formatted how the top-level message was formatted. So now we actually handle that, and have an additional notes field in the message/diagnostic that we parse that additional information into so that it can be treated distinctly from the top-level error message.

[Jake-Shadle]

(this also made me realize the gets_error_message test had already shown there was this difference between installed and compiled, but apparently I went for the lazy route and just chopped off the end that was different, oops)

[eddyb]

Thanks for fixing this! Left some review comments (nothing super important though, I don't think).