Reworked Indexes, Error Messages and overall Usability

Indexes are now handled in a "Indexes"-Column in the mapping. Each number corresponds to one occurence of <> in the Flat-Path.

Error Messages often contain the Status Code and Text given by the EHRBase. This helps a lot with identifying content related issues with the mapping / template.

Some Error Cases are now catched e.g. by checking for existence of needed columns and giving proper feedback.