use custom exceptions to improve error handling

This patch adds the KasUserError exception class to distinguish between
internal kas exceptions and user or configuration errors. Exceptions
previously raised on user errors are ported over by deriving
KasUserError. In case of user errors, only the exception message is
shown, but no stacktrace. This makes it easier for users to locate the
issue as the reason is now stated in the last line of the output.

Kas internal exceptions are not subject to this change to help the
developers to find the root cause more easily.

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

kas/kas.py

@@ -21,7 +21,9 @@
 # SOFTWARE.
 """
     This module is the main entry point for kas, setup tool for bitbake based
-    projects
+    projects. In case of user errors (e.g. invalid configuration, repo fetch
+    failure) KAS exits with error code 2, while exiting with 1 for internal
+    errors. For details on error handling, see :mod:`kas.kasusererror`.
 """
 
 import argparse
@@ -32,6 +34,7 @@ import logging
 import signal
 import sys
 import os
+from .kasusererror import KasUserError
 
 try:
     import colorlog
@@ -173,6 +176,9 @@ def main():
 
     try:
         kas(sys.argv[1:])
+    except KasUserError as err:
+        logging.error('%s', err)
+        sys.exit(2)
     except Exception as err:
         logging.error('%s', err)
         traceback.print_exc()