Merge #15631: qa: mininode: Clearer error message on invalid magic bytes

fab0a68 qa: mininode: Clearer error message on invalid magic bytes (MarcoFalke)

Pull request description:

  Old message:
  ```
  ValueError: got garbage b'\xfa\xbf\xb5\xdafeefilter\x00\x00\x00\x08\x00\x00\x00\xe8\x0f\xd1\x9f\xe8\x03\x00\x00\x00\x00\x00\x00'
  ```

  New message:
  ```
  ValueError: magic bytes mismatch: b'\x00\x11"2' != b'\xfa\xbf\xb5\xdapong\x00\x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x97\xe6\x04\xd2\xff\x00\x00\x00\x00\x00\x00\x00'
  ```

ACKs for commit fab0a6:
  laanwj:
    Much better message, utACK fab0a68

Tree-SHA512: 55854ef93d79c4f3219440e367fbcdf5db7b54c6530db0087fa8f8d9e786dac713f05da6f6c4c3cf29e18d07b4472f96a8ed894d66318e08cf8f208252199b70

Author: MarcoFalke

test/functional/test_framework/mininode.py

@@ -171,7 +171,7 @@ def _on_data(self):
                 if len(self.recvbuf) < 4:
                     return
                 if self.recvbuf[:4] != self.magic_bytes:
-                    raise ValueError("got garbage %s" % repr(self.recvbuf))
+                    raise ValueError("magic bytes mismatch: {} != {}".format(repr(self.magic_bytes), repr(self.recvbuf)))
                 if len(self.recvbuf) < 4 + 12 + 4 + 4:
                     return
                 command = self.recvbuf[4:4+12].split(b"\x00", 1)[0]