Improve import error messages

[mikehaertl]

I tried to import a quite large openapi spec and all I get is this error:

Error while scanning for resources to import:Cannot convert undefined or null to object

As the file is ~220 KB it's pretty hard to figure out, where the problem could be. It would be helpful if there would at least be some pointer to the location in the file.

Unfortunately the file is from a private project so I can't share it.