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>

kas/kasusererror.py

@@ -43,3 +43,28 @@ class KasUserError(Exception):
     User or input error. Derive all user error exceptions from this class.
     """
     pass
+
+
+class CommandExecError(KasUserError):
+    """
+    Failure in execution of a shell command. The `forward_error_code` parameter
+    can be used to request the receiver of the exception to `sys.exit` with
+    that code instead of a generic one. Only use this in special cases, where
+    the return code can actually be related to a single shell command.
+    """
+    def __init__(self, command, ret_code,
+                 forward_ret_code=False):
+        self.ret_code = ret_code
+        self.forward = forward_ret_code
+        message = ["'{}'".format(c) if ' ' in c else c for c in command]
+        super().__init__('Command "{}" failed with error {}'
+                         .format(' '.join(message), ret_code))
+
+
+class ArgsCombinationError(KasUserError):
+    """
+    Invalid combination of CLI arguments provided
+    """
+    def __init__(self, message):
+        super().__init__('Invalid combination of arguments: {}'
+                         .format(message))