[val] Add extra context to error messages. (#1600)

[val] Add extra context to error messages.

This CL extends the error messages produced by the validator to output the
disassembly of the errored line.

The validation_id messages have also been updated to print the line number of
the error instead of the word number. Note, the error number is from the start
of the SPIR-V, it does not include any headers printed in the disassembled code.

Fixes #670, #1581