Improve error message when Phabricator fails to load a class.

Summary: add references to documentation into the error message.

Test Plan: modify a class name in a file to trigger the error message.
For example, modify ""ManiphestTaskPriority"" to ""FOO"" in
ManiphestTaskSummaryView.php in directory
phabricator/src/applications/maniphest/view/tasksummary/.

Reviewers: epriestley

CC:

Differential Revision: 49