Dashboard cleanup: Remove globals g_buildersThatFailedToLoad and g_staleBuilders

Dashboard cleanup: Remove globals g_buildersThatFailedToLoad and g_staleBuilders
https://bugs.webkit.org/show_bug.cgi?id=106356

g_buildersThatFailedToLoad and g_staleBuilders were globals defined in
dashboard_base, assigned by Loader, and used only by dashboard_base to
create error messages.  Moved the variables to be privates on the Loader
object, moved error message creation to _getLoadingErrorMessages on the
Loader object, and now pass the errors back to dashboard base via the
resourceLoadingComplete callback.

Also removed the now unused clearError function, it was only being used
by unit tests to clean up global state.

Reviewed by Dirk Pranke.

* TestResultServer/static-dashboards/dashboard_base.js:
(resourceLoadingComplete):
* TestResultServer/static-dashboards/flakiness_dashboard_unittests.js:
* TestResultServer/static-dashboards/loader.js:
(.):
* TestResultServer/static-dashboards/loader_unittests.js:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@139097 268f45cc-cd09-0410-ab3c-d52691b4dbfc