[release] prepping v0.5.0
[backend] handle err === null (dummy row)
[backend] correctly update local source urls
[release] prepping v0.5.1
[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
[project] enabled strict mode in all files

forgot to add "use strict" to js/data/merger.js and js/data/interpreter.js
[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
[options] errored manifests now truly link to the spreadsheet

40f01c7bd332f100dcc56d6f88db930729022383 applied the correct fix in the wrong location
this has been reverted and the fix applied in the correct location
[options] fixed bug where faulty manifests still triggered "success" response

linked to #31, but not really the same
[release] prepping v0.5.3
Merge branch 'version-0.5' into develop