[backend] improved data merging code
[backend] keep highest known player level

Merge player levels so the highest known level is retained
instead of the last defined level.
fixes #30
[options] errored manifests now also link to the spreadsheet
[options][backend] fixed warning bug

There was a bug that resulted in the extension interpreting an empty set of errors
as "non-fatal errors were thrown".

This commit fixes #31
[release] prepping v0.5.2
[release] v0.5.2