Re-raise exceptions instead of sys.exit

If we call sys.exit(), code in finally block won't be executed.
Test: th

Change-Id: I1f743c9385218742def3b925427942cb1d5bd031

tools/releasetools/sign_target_files_apks.py

@@ -1383,6 +1383,6 @@ if __name__ == '__main__':
     main(sys.argv[1:])
   except common.ExternalError as e:
     print("\n   ERROR: %s\n" % (e,))
-    sys.exit(1)
+    raise
   finally:
     common.Cleanup()