refactor: port all sys.exit over to kas exceptions

This patch replaces all direct invocations of sys.exit outside of the
main invocation to KasUserError based exceptions. By that, only one
method for returning is used and return codes can be handled
consistently. In addition, this makes it possible to handle specific
errors differently.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_refspec.py

@@ -25,6 +25,7 @@ import pytest
 import shutil
 from kas import kas
 from kas.libkas import run_cmd
+from kas.repos import RepoRefError
 
 
 def test_refspec_switch(changedir, tmpdir):
@@ -95,5 +96,5 @@ def test_url_no_refspec(changedir, tmpdir):
     tdir = str(tmpdir / 'test_url_no_refspec')
     shutil.copytree('tests/test_refspec', tdir)
     os.chdir(tdir)
-    with pytest.raises(SystemExit):
+    with pytest.raises(RepoRefError):
         kas.kas(['shell', 'test4.yml', '-c', 'true'])