Handle not being a git repo better

[kathenas]

Handle not being a git repo better.

Claiming 'fatal' confuses and should be more elegant in how this component is passed or failed.

[philwyett@ks-kenobi bcunit-5.2.0]$ cmake .
-- The C compiler identification is GNU 8.5.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/lib64/ccache/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Found Git: /usr/bin/git (found suitable version "2.31.1", minimum required is "1.7.10") 
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
-- Configuring done
-- Generating done
-- Build files have been written to: /home/philwyett/Development/temporary/bcunit-5.2.0