pyln-testing: Just warn if we get an unexpected fee request

This was breaking a couple of tests if the pyln version was not synced up with
`lightningd`, so now we just warn (these are then collected when running in
pytest and highlighted).

contrib/pyln-testing/pyln/testing/utils.py

@@ -21,6 +21,7 @@ import struct
 import subprocess
 import threading
 import time
+import warnings
 
 BITCOIND_CONFIG = {
     "regtest": 1,
@@ -871,9 +872,10 @@ class LightningNode(object):
             elif params == [100, 'ECONOMICAL']:
                 feerate = feerates[3] * 4
             else:
-                raise ValueError("Don't have a feerate set for {}/{}.".format(
+                warnings.warn("Don't have a feerate set for {}/{}.".format(
                     params[0], params[1],
                 ))
+                feerate = 42
             return {
                 'id': r['id'],
                 'error': None,