Improve exception logic

[tomasssalles]

Decide on which custom exception classes we want to have and when to use them.

[tomasssalles]

From old comments in the source code:

TODO: Which exceptions do we want?

• Invalid type-hint?
• Wrong type value (default or from yaml)?
• Default secret?
• Wrong class syntax?
• Do we even want to keep the ones we already have? When would people use them?
  When would people catch a config exception and react without crashing?
  Isn't a config exception something that should just prevent an app from starting?
  Specific exceptions can help with meaningful names, but we can get the same from
  good error messages, can't we?