Converter/core: add errors to check result

[jonathanlukas]

Is your feature request related to a problem? Please describe.

Currently, there is no feedback in the API whether there were problems.
By adding the error to the check result, controllers consuming the API can return them as well.

Describe the solution you'd like

An additional field in the check result that adds information for each occurred error.

Information:

bpmn element
element tree
visitor name

error message
stacktrace

Errors should be grouped by the upper information. The actual error details are atomic.

Describe alternatives you've considered

Additional context