Trim overlong source fragments in error messages

[ChrisDodd]

• too-long source lines (particularly those that come from preprocessor macros) tend to give unreadble error messages. Ideally they should be trimmed based on the width of the output terminal.

This code is a compromise that defaults the trim width to 100 when the useMarker flag is defaulted to true (the way this is used for user-facing error messages) and to 0 (no trimming) when the flag is set explicit (generally to false) -- one place this is used is in some logging code.

[vlstill]

Nice addition to make the messages easier to read, thank you!

Alternative to hadrcoded 100 would be to default to the current terminal width (with a fallback to 100 or 80), but I guess detecting that every time a source info is being formatted would not be practical.

[ChrisDodd]

Alternative to hardcoded 100 would be to default to the current terminal width (with a fallback to 100 or 80), but I guess detecting that every time a source info is being formatted would not be practical.

If we default to the terminal width, we'd want a way to override that with a command line arg (and use that arg in ctest) -- otherwise you'd see spurious test failures due to using different terminal sizes. There's also the question of how to portably determine terminal size -- linux/windows are quite different and we've so far avoided putting OS-specific stuff in the compiler. Best compromise would probably be to use the COLUMNS environment variable, which both can use (and we could override in run_test.py).