Error clarity: Document insertion failed unexpectedly (due to abstract subdocument type)

[hoijnet]

Describe the bug
When a subdocument has the abstract flag on it, it (correctly) prevents an instance of it to be created.

The error message is very hard to track down and I was worried about an internal issue in TerminusDB.

The schema checker should be improved to report the errors of abstract subdocuments properly and give a better error message.

To Reproduce
Steps to reproduce the behavior:

1. Create an abstract subdocument type
2. Add it to a type
3. Create a document of the type
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

{
  "name": "Server request error",
  "message": "Document insertion failed unexpectedly",
  "data": {
    "@type": "api:InsertDocumentErrorResponse",
    "api:error": {
      "@type": "api:DocumentInsertionFailedUnexpectedly",
      "api:document": {

Info (please complete the following information):

• OS: DFRNT

Additional context
N/A