Refactors cli to avoid multi-type returns (normal return + None). Adds exceptions for errors.

cli/exceptions.py

@@ -0,0 +1,22 @@
+class InvalidParameter(ValueError):
+    """Raised when a command line parameter is invalid (either missing or wrong)"""
+
+    def __init__(self, msg, **kwargs):
+        self.msg = msg
+        self.kwargs = kwargs
+
+
+class InvalidKey(Exception):
+    """Raised when there is an error loading the keys"""
+
+    def __init__(self, msg, **kwargs):
+        self.reason = msg
+        self.kwargs = kwargs
+
+
+class TowerResponseError(Exception):
+    """Raised when the tower responds with an error"""
+
+    def __init__(self, msg, **kwargs):
+        self.reason = msg
+        self.kwargs = kwargs