Better POST api validation

[savpek]

There have two issues:

• When end user posts new PDF there can be empty array for rows, it doesn't generate anything and returns empty array. Its hard to catch.
• When user tries to fix previous and adds any object in language like C# to request, it could end in form like [ 1 ], array of number 1, this isn't accepted and error is something like internal server error.

Solution:

• Modify api not to accept empty array and return bad request with human readable information what to fix if so.
• If invalid row object is sent (like not parseable as object), return bad request with good information about issue.