Improve error handling for unresolved nested schemas

[jordanpadams]

Describe the bug When using a catalog file, if a schema cannot be resolved, the error handling needs some improvement. For several cases, it throws erroneous or vague error messages.

To Reproduce CASE 1

1. Remove src/test/resources/github87/pds/v1/PDS4_PDS_1900.xsd from your local version of validate
2. Try to run integration test testGithub87
3. Take a look at target/test/report_github87_1.json to see the bogus error message:

    Begin Schema: http://pds.nasa.gov/pds4/pds/v1/PDS4_PDS_1900.xsd
      ERROR  [error.label.schema]   line 1, 50: White spaces are required between publicId and systemId.
    End Schema: http://pds.nasa.gov/pds4/pds/v1/PDS4_PDS_1900.xsd

CASE 2 See #116 . If the catalog resolver cannot resolve a nested schema properly, it only throws the error:

ERROR  [error.label.schema]   line 1, 1: Content is not allowed in prolog.

We should do some checks to see if it resolves, and if not, throw the proper error message.