Merge #13707: tests: Add usage note to check-rpc-mappings.py

03a2d68010 Tests: add usage note to check-rpc-mappings.py (Mason Simon)

Pull request description:

  This test would previously fail without a user-friendly warning message, if invoked with no arguments.

  Test plan:
  ```
  bitcoin @_@$ python3 test/lint/check-rpc-mappings.py
  Usage: test/lint/check-rpc-mappings.py ROOT-DIR
  bitcoin @_@$ echo $?
  1
  bitcoin @_@$ python3 test/lint/check-rpc-mappings.py .
  * Checking consistency between dispatch tables and vRPCConvertParams
  bitcoin @_@$ echo $?
  0
  ```

Tree-SHA512: 0b7a94125d18ba5dbf3c3281a4af60718d2e495bf7645d7c2a3e1a2a1d80cffcea1383c90247186728037f1f2b38de75d0f59ccf7f792d69edd33884f3698c07

test/lint/check-rpc-mappings.py

@@ -90,6 +90,10 @@ def process_mapping(fname):
     return cmds
 
 def main():
+    if len(sys.argv) != 2:
+        print('Usage: {} ROOT-DIR'.format(sys.argv[0]), file=sys.stderr)
+        sys.exit(1)
+
     root = sys.argv[1]
 
     # Get all commands from dispatch tables