Builder should shorten log messages

[Reinmar]

Followup of https://github.com/ckeditor/ckeditor5-core/issues/38.

All usages of log.error(), log.warning(), CKEditorError() must be processed and the messages should be removed (only error names should be left). Additionally, we may decide to add some validation (may be built into this step) that error messages have the correct format.

Note: Theres' no support yet for placeholders in the error messages (they were mentioned in ckeditor/ckeditor5-design#14 (comment)). I don't think we must have, so I didn't want to unnecessarily complicate things.

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[CKEditorBot]

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