[RTADPA BW Tests] Report error on MR even if comparison reference is unavailable

See: lhcb/Moore!2834 (comment 7475126)

Currently, if the nightlies (lhcb-master) is broken, when testing with a PR label, such as PR/AllenInMoore_hlt1_bandwidth there won't be any report on the MR regardless of if the lhcb-master-mr test passed/failed. This is because of the error reported below.

Adding some more logic to the Handler so that it posts a comment about the status of the new test in this situation seems like a useful quality of life improvement.

2024-01-11 23:12:48,028 - Dashboard - WARNING - no couchdb credentials found
2024-01-11 23:12:48,056 - handlers.utils.dashboard - INFO - Determined ref build to be lhcb-master.2264
2024-01-11 23:12:48,057 - Dashboard - WARNING - no couchdb credentials found
2024-01-11 23:12:48,453 - collectRunResults.py - ERROR - Handler exception: [Errno 2] No such file or directory: '/eos/lhcb/storage/lhcbpr/www/UpgradeRateTest/BandwidthTest_lhcb-master.2264_AllenInMoore_hlt1_bandwidth_x86_64_v3-centos7-gcc12+detdesc-opt+g_2024-01-11_04:28:44_+0100/hlt1__rates_for_all_lines.csv'
Traceback (most recent call last):
  File "/home/lhcbpr/jenkins/perf-j16/workspace/nightly-builds/tests/./LHCbPR2HD/collectRunResults.py", line 389, in main
    currentHandler.collectResultsExt(
  File "/home/lhcbpr/jenkins/perf-j16/workspace/nightly-builds/tests/LHCbPR2HD/handlers/BandwidthTestHandler.py", line 570, in collectResultsExt
    ) = get_info_from_comparison_build(
  File "/home/lhcbpr/jenkins/perf-j16/workspace/nightly-builds/tests/LHCbPR2HD/handlers/BandwidthTestHandler.py", line 111, in get_info_from_comparison_build
    dfs = {
  File "/home/lhcbpr/jenkins/perf-j16/workspace/nightly-builds/tests/LHCbPR2HD/handlers/BandwidthTestHandler.py", line 112, in <dictcomp>
    csv_name: pd.read_csv(os.path.join(directory, csv_name))
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/util/_decorators.py", line 311, in wrapper
    return func(*args, **kwargs)
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/io/parsers/readers.py", line 680, in read_csv
    return _read(filepath_or_buffer, kwds)
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/io/parsers/readers.py", line 575, in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/io/parsers/readers.py", line 933, in __init__
    self._engine = self._make_engine(f, self.engine)
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/io/parsers/readers.py", line 1217, in _make_engine
    self.handles = get_handle(  # type: ignore[call-overload]
  File "/cvmfs/lhcbdev.cern.ch/conda/envs/default/2022-03-16_10-04/linux-64/lib/python3.9/site-packages/pandas/io/common.py", line 789, in get_handle
    handle = open(
FileNotFoundError: [Errno 2] No such file or directory: '/eos/lhcb/storage/lhcbpr/www/UpgradeRateTest/BandwidthTest_lhcb-master.2264_AllenInMoore_hlt1_bandwidth_x86_64_v3-centos7-gcc12+detdesc-opt+g_2024-01-11_04:28:44_+0100/hlt1__rates_for_all_lines.csv'