Improve exit code implementation

Unnecessary dynamic GitReviewException.EXIT_CODE attribute
lookup refactored into static to benefit static analyzers.

Default process exit code changed from 127 (command not found error)
to 1 (general error)

Change-Id: I1fcb583a740bf32c4427a587e208d099712a7bc4

git_review/cmd.py

@@ -75,7 +75,7 @@ class colors(object):
 
 
 class GitReviewException(Exception):
-    EXIT_CODE = 127
+    EXIT_CODE = 1
 
 
 class CommandFailed(GitReviewException):
@@ -1704,7 +1704,7 @@ def main():
         _main()
     except GitReviewException as e:
         print(e)
-        sys.exit(getattr(e, 'EXIT_CODE', -1))
+        sys.exit(e.EXIT_CODE)
 
 
 if __name__ == "__main__":

releasenotes/notes/fix-exit-code-5ab7f6314976319f.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes ``gitreview`` process exit code to align with general
+    UNIX process exit code conventions. Specifically, default exit
+    code changed to ``1`` which stands for "general error".