Split up application error handling

[askvortsov1]

Fixes #2742

• Use a catch instead of a then with a vacuous onfulfilled
• Instead of setting a default errorHandler that throws the error, just run the fallback handler if no error handler is explicitly provided
• Split requestErrorCatch and requestErrorDefaultHandler into separate methods.
• Clean up some messy duplicate code with formattedErrors and content on 422 errors
• Properly typehint AlertAttrs