Get file size for download from the RawFileStore instead of the DB

[kkoz]

Similar to #180
This PR is to fix the issue where attempting to download a file with a null size in originalfile table results in the following:

  File "OMERO/bin/omero", line 130, in <module>
    rv = omero.cli.argv()
  File "OMERO/lib/python/omero/cli.py", line 1620, in argv
    cli.invoke(args[1:])
  File "OMERO/lib/python/omero/cli.py", line 1095, in invoke
    stop = self.onecmd(line, previous_args)
  File "OMERO/lib/python/omero/cli.py", line 1172, in onecmd
    self.execute(line, previous_args)
  File "OMERO/lib/python/omero/cli.py", line 1254, in execute
    args.func(args)
  File "OMERO/lib/python/omero/plugins/download.py", line 66, in __call__
    client.download(orig_file, target_file)
  File "OMERO/lib/python/omero/clients.py", line 863, in download
    if block_size > ofile.size.val:

If, however, we retrieve the size from the RawFileStore instead of the DB record, we can get a size for the file even if the DB field is null.

[joshmoore]

Thanks, @kkoz. You might want to also copy over the ClientError handling from #180.

[jburel]

Wrote a test (I will open a PR against openmicroscopy)
Without this PR:

E           AttributeError: 'NoneType' object has no attribute 'val'

With this PR
the test is green

[jburel]

Client side handling has been copied over
Merging