Added debug log for process timeout

I added this because I otherwise had no idea why a particular linter was not returning on a large file
Merge pull request #200 from pjking07/contrib/log-process-timeout

Added debug log for process timeout
More informative timeout logging

Test Plan:
Changed multiplier from 1000 to 10 and reloaded window. Saw linters time out.
Use the icon-right entity for gutter markers

Fixes #202.

Also is better because it doesn't make the gutter wider. Note that this
occupies the same spot as the fold marker, but that only shows up on hover
anyway, and it still appears.

Another change here is to begin using @text-color-warning and @text-color-error
which are provided by the theme.

Test Plan:
Looked at both error and warning level lint messages.
Remove dependency on Font Awesome

Test Plan:
Reloaded window; lint messages still appear
:lipstick: get rid of long lines
Use theme colours instead of hardcoded ones

Test Plan:
Loaded window with both kinds of lint messages and it looked reasonable
:memo: update changelog
Prepare 0.5.18 release