Use Whoops as error handler

[Schlaefer]

Whoops has matured to a point that it is better than Phile's development error handler, so I think it's time to switch. This PR:

• Replaces Phile's custom development error handler with Whoops.
• Removes the display_errors config option.
• Adds a config option to the error-log handler to specify a error.log path.