Include errors in the info field of the json output

[ResistantBear]

Include errors in the info field of the json output

Closes #185

Description

No longer includes "ERROR" metrics into the list of metrics in the output .json-file, but include errors in the "info" field of the .json-file. Refactors error handling accordingly.

The user is now informed about if parsing a syntax tree failed or if calculating some metrics failed for a particular file.

Definition of Done

A PR is only ready for merge once all the following acceptance criteria are fulfilled:

• [x] All TODOs related to this PR have been closed
• [x] There are automated tests for newly written code and bug fixes
• [x] All bugs discovered while working on this PR have been submitted as issues (if not already an open issue)
• [x] Documentation (README.md, UPDATE_GRAMMARS.md) has been updated

[clemens-mw]

I love the idea of adding a new class for ErrorFiles!!