Create mechanism to inform user about common errors

[LEDfan]

cc @jancborchardt

First implementation in 8c260f7f88d8365327b5ea62a14a0a0a1076b1f7

[LEDfan]

[LEDfan]

TODO:

• [x] make it look like other ownCloud pages, so the user know it's a page about ownCloud
• [x] add the following errors to it:
  • [x] 404 shared contact not found
  • [x] 404 contact not found
  • [x] Common DB problems
  • [x] ldap_user problems
  • [ ]

[LEDfan]

@jancborchardt the most common errors are now caught. The error page looks like this:

[jancborchardt]

I would say that’s a prime example of great error handling from an app. :) cc @DeepDiver1975

Just one thing: Instead of making it a white box, use the class error for the container. That should format it nicely.