[Fleet] Improve error messages when uploading invalid package

[juliaElastic]

Fleet added better support for uploaded packages here: #148599
While testing it, we encountered an error when uploading an invalid package, which is expected.

The error message should be improved, so it is easier to track down which part of the package is invalid.
In this example the required property was missing from an ingest pipeline.

{"statusCode":500,"error":"Internal Server Error","message":"parse_exception\n\tRoot causes:\n\t\tparse_exception: [pattern] required property is missing"}%

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)