Error Handling Improvements

Addition of server-side (C#) Airbrake error reporting to complement existing client-side (JS) Airbrake reporting.
Distinguishing JS errors from C# errors in Airbrake by appending " (JS)" to environment entries so can distinguish between both in filters.
New AppKey toggle to enable/disable client-side Airbrake reporting. Useful as service had a recent outage. (See http://status.airbrake.io/.)
New AppKey toggle to enable/disable server-side Airbrake reporting. Useful as service had a recent outage. (See http://status.airbrake.io/.)
New friendly generic error page created and wired up for uncaught server-side errors. Can view directly at /error.html to see what looks like.
Web.config changes so developers on same local box see real error, and everyone else (real users) see the generic error page.

The generic error page is a better choice for end-users since it:

Hides the real server-side error, which can otherwise be examined and exploited by hackers and malicious crawlers.
Is a friendlier experience with some options for next steps, and is more intelligible for the end-user than a technical error.
AppKey Configuration Changes
- New key "Airbrake.TrackServer" can be set to true/false to indicate if we want to use Airbrake to track server-side C# errors.
- New key "Airbrake.TrackClient" can be set to true/false to indicate if we want to use Airbrake to track client-side JS errors.
- New key "Airbrake.ProjectId" replaces old key "AirbrakeProjectId". Existing values should be migrated.
- New key "Airbrake.ApiKey" replaces old key "AirbrakeProjectKey". Existing values should be migrated.
- New key "Airbrake.Environment" replaces old key "AirbrakeEnvironmentName". Existing values should be migrated.

alterm4nn / ChronoZoom