Move PhaseTracker to its own file

One "class" per file is a good thing I think
Move ErrorTracker to its own file

One thing per file is a nice thing
Generalize ErrorTracker => MessageTracker

This will make it possible to use it to track breadcrumbs too
Add prettyBreadcrumb helper

For better timeout reporting
Report last breadcrumb (if there is one) on test timeout
Exports MessageTracker in the public API

That will be useful for anyone who wants to do something similar to
what ErrorDetail does but with different formatting.
Merge pull request #53 from pereckerdal/error-detail-breadcrumbs

Report the last breadcrumb on timeout