Check that file exists before uploading to Backblaze

[grahams]

If the file on disk does not exist, we shit the bed with a stack trace. Also, a failure here should not crash the application.

Line of interest:

trackupdate/plugins/BackblazeTarget.py

Line 78 in 8dd9bc3

self.bucket.upload_local_file(

Stacktrace:

Traceback (most recent call last):
  File "./trackupdate.py", line 333, in updateTrack
    plugin.logTrack(track, startTime)
  File "/Users/grahams/src/trackupdate/plugins/BackblazeTarget.py", line 78, in logTrack
    self.bucket.upload_local_file(
  File "/usr/local/anaconda3/envs/trackupdate/lib/python3.8/site-packages/logfury/v0_1/trace_call.py", line 84, in wrapper
    return function(*wrapee_args, **wrapee_kwargs)
  File "/usr/local/anaconda3/envs/trackupdate/lib/python3.8/site-packages/b2sdk/bucket.py", line 402, in upload_local_file
    upload_source = UploadSourceLocalFile(local_path=local_file, content_sha1=sha1_sum)
  File "/usr/local/anaconda3/envs/trackupdate/lib/python3.8/site-packages/b2sdk/transfer/outbound/upload_source.py", line 84, in __init__
    raise InvalidUploadSource(local_path)
b2sdk.exception.InvalidUploadSource: Invalid upload source: /Users/grahams/Resilio Sync/Public/programme/covers/20210125.jpg

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./trackupdate.py", line 381, in <module>
    trackUpdate = TrackUpdate(sys.argv[1:])
  File "./trackupdate.py", line 190, in __init__
    self.liveLoop()
  File "./trackupdate.py", line 231, in liveLoop
    self.processCurrentTrack(track)
  File "./trackupdate.py", line 318, in processCurrentTrack
    self.updateTrack(track, self.startTime)
  File "./trackupdate.py", line 335, in updateTrack
    logging.error(plugin + ": Error trying to update track")
TypeError: unsupported operand type(s) for +: 'BackblazeTarget' and 'str'