Description of problem:
Importing a JSON definition never fails even if this definition is invalid. In that case, Designer imports nothing (empty process).

Version-Release number of selected component (if applicable):
6.0.0. ER3

How reproducible:


Steps to Reproduce:
1. Create new process (Start -> Task -> End)
2. View its JSON definition and copy it (Ctrl+A, Ctrl+C) 
3. Open import dialog for JSON
4. Insert JSON definition and delete any character ('}', ']', etc.)

Actual results:
Alert "Importing..." is shown together with confirmation dialog "Replace existing model?". If you answer 'YES' then the empty process is shown.


Expected results:
Same behaviour as for BPMN2:
  - Alert "Failed to import JSON"
  - Alert with more specific message ("missing character", etc.)
  - no confirmation dialog
  - previous process is shown (without changes)


Additional info: