fix(registerplugin): better error on type

[SanderElias]

When given the wrong type for a plugin, throw a proper explaination.

closes #219

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

The current error message isn't explanatory enough

What is the new behavior?

throw an error like this:

--------------
  Type "tourer" is not a known plugin type for registering plugin "myPluginName".
  The first parameter of registerPlugin needs to be one of: 'fileHandler', 'router', 'render'
--------------

Does this PR introduce a breaking change?

• Yes
• No