Improve error message when getting output from cancelled/failed jobs

[wkoszek]

Currently when running bsubio cat on a cancelled job, the error message is not helpful:

Error: failed to get job output: HTTP 400

Expected Behavior

The error message should indicate the actual reason, such as:

• Job was cancelled
• Job failed
• Job is still pending/claimed
• Job has no output available

Implementation

Parse the response body from the server to extract the real error message and display it to the user instead of just showing the HTTP status code.

This applies to:

• bsubio cat command
• Potentially other commands that show generic HTTP error codes