Handle RelativeUrlWithoutBases error

[flit]

0000262 I Updating pack index... [pack_cmd]
RelativeUrlWithoutBases (804/838)
0002770 C Error: b'\nrelative URL without a base' [__main__]
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/pyocd/__main__.py", line 161, in run
    status = cmd.invoke()
  File "/usr/local/lib/python3.10/dist-packages/pyocd/subcommands/pack_cmd.py", line 112, in invoke
    cache.cache_descriptors()
  File "/usr/local/lib/python3.10/dist-packages/cmsis_pack_manager/__init__.py", line 299, in cache_descriptors
    pdsc_index = self._call_rust_update(progress_fn)
  File "/usr/local/lib/python3.10/dist-packages/cmsis_pack_manager/__init__.py", line 244, in _call_rust_update
    return self._poll_rust_update(poll_obj, on_tick_fn)
  File "/usr/local/lib/python3.10/dist-packages/cmsis_pack_manager/__init__.py", line 247, in _poll_rust_update
    with _RaiseRust():
  File "/usr/local/lib/python3.10/dist-packages/cmsis_pack_manager/__init__.py", line 36, in __exit__
    raise Exception(ffi.string(maybe_err))
Exception: b'\nrelative URL without a base'

Actual cause not known. Regardless, the error should be caught so the bad URL can be ignored and the index download doesn't stop.

See pyocd/pyOCD#1506 for original issue