Introduce base exception classes

GitReviewException:

Handle also exceptions that are
not related to the failed external
commands at toplevel.

ChangeSetException:

For exceptions not related to the
execution of the external command;
such an exception is associated
with a change set identifier.

Change-Id: I8d165d1dfee5ec4e61487565e90b033fb6f58acf

git-review

@@ -59,7 +59,11 @@ class colors:
     reset = '\033[0m'
 
 
-class CommandFailed(Exception):
+class GitReviewException(Exception):
+    pass
+
+
+class CommandFailed(GitReviewException):
 
     def __init__(self, *args):
         Exception.__init__(self, *args)
@@ -78,6 +82,16 @@ The following command failed with exit code %(rc)d
 -----------------------""" % self.quickmsg
 
 
+class ChangeSetException(GitReviewException):
+
+    def __init__(self, e):
+        GitReviewException.__init__(self)
+        self.e = str(e)
+
+    def __str__(self):
+        return self.__doc__ % self.e
+
+
 def run_command_status(*argv, **env):
     if VERBOSE:
         print(datetime.datetime.now(), "Running:", " ".join(argv))
@@ -861,6 +875,6 @@ def main():
 if __name__ == "__main__":
     try:
         main()
-    except CommandFailed, e:
+    except GitReviewException, e:
         print(e)
         sys.exit(e.EXIT_CODE)

git-review.1

@@ -108,6 +108,18 @@ defaultremote=review
 defaultrebase=0
 .Ed
 .Pp
+.Sh DIAGNOSTICS
+.Pp
+Normally, exit status is 0 if executed successfully.
+Exit status 1 indicates general error, sometimes more
+specific error codes are available:
+.Bl -tag -width 999
+.It 2
+Gerrit
+.Ar commit-msg
+hook could not be successfully installed.
+.El
+.Pp
 .Sh EXAMPLES
 To fetch a remote change number 3004:
 .Pp