Pretty exit when no files found

[toby-bro]

Just a tiny PR to make the output a tiny bit more explicit and simple when the user gives in an input dir that has no coverage files in it

before:

[0.037s] [info]: Found 0 coverage files (.gcda)
[0.003s] [info]: Spawned 0 gcov processes, each processing at most 5 coverage files
Traceback (most recent call last):
  File "/usr/local/bin/fastcov", line 1071, in <module>
    main()
  File "/usr/local/bin/fastcov", line 1045, in main
    fastcov_json = getGcovCoverage(args)
                   ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/bin/fastcov", line 891, in getGcovCoverage
    fastcov_json = processGcdas(args, coverage_files, gcov_filter_options)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/bin/fastcov", line 368, in processGcdas
    base_fastcov = fastcov_jsons.pop()
                   ^^^^^^^^^^^^^^^^^^^
IndexError: pop from empty list

after:

[0.038s] [info]: Found 0 coverage files (.gcda)
[0.000s] [error]: No coverage files found in directory '.'

[toby-bro]

Sorry I hadn't checked the CI.
Moved the check as requested.
Ran the CI locally, the tests seem to pass now.

[RPGillespie6]

Thanks for the fix!