Handle errors

[moonglum]

When an error occurs while compiling templates etc., we should show an error page when in watch mode and abort with a human readable error when in compile mode.

Also:

• Error: Cannot find module '…/aiur.config.js'
• Error: ENOENT: no such file or directory, open '…/components/…/index.md'

[FND]

Potentially relevant: If an error occurs while executing complate views, we wanna make the respective bundle's source code available (somehow - ideally by parsing the exception and showing the respective lines with context).