Log errors for upload directly

[alneberg]

Errors are now logged more or less directly after failure upon exiting the verify_proceed wrapper. Since multiple threads could be appending lines to the log, I changed the format of the log so that each line is valid json but the entire file is unfortunately not. Parsing should be easy enough just reading line by line.

I ran 32 threads (on my laptop though) and multiple threads appending to the file did not cause any issues, but I'm not sure it's a recommended praxis. Safer would be to have a single thread taking care of the logging or keeping a global lock variable in the owning class perhaps.

However, I think there might be better ways of making sure that the bucket and database is in sync which seems to be the ultimate goal of this log.

[i-oden]

Yes! This is exactly what I had in mind.