Don't try and rewrite XPI on disk if it's already there and correct

This fixes issues if you have two projects running tests running at the same time on one machine (e.g. in CI)
Add happy-path test, for case where there are no errors
General tidy up and refactoring of JavaScriptError

Converted to equivalent linq expression, and made unnecessarily public
constructor private.
Mark as version 0.6.0
Merge pull request #6 from pimterry/checkBeforeWriting

Don't try and rewrite the XPI on disk if it's already there and correct
Merge pull request #7 from pimterry/general-tidyup

Some general last bits of tidyup, and a new version number