Add support for CodeChecker warning/error reasons

[Discookie]

Fixes (partially) #137

CodeChecker is assumed to output a summary of its analysis in a user-readable manner, on its last stderr line.

This reason is now displayed on the sidebar, if available. Also added support for differentiating between CodeChecker warnings/errors.

(The full reason is of course shown on hover.)

[Discookie]

Reference for the last-line assumption on the CodeChecker side: Ericsson/codechecker#4222