Fix: Improve error messages for cancelled/failed jobs (fixes #131)

[wkoszek]

Summary

Parse server response body to extract actual error messages instead of showing generic HTTP status codes. Provides clearer feedback when jobs are cancelled, failed, or otherwise unavailable.

Changes

• Add JSON error response parsing in cat command
• Add JSON error response parsing in submit command
• Show actual error message from server when available
• Fall back to showing response body if JSON parsing fails
• Import encoding/json and io packages

Example

Before

$ bsubio cat <cancelled-job-id>
Error: failed to get job output: HTTP 400

After

$ bsubio cat <cancelled-job-id>
Error: failed to get job output: job was cancelled

Implementation

When receiving non-200 status from GetJobOutput:

1. Read response body
2. Try to parse as JSON with {"error": "message"} format
3. If successful, show the error message
4. Otherwise, show HTTP status + raw response body

Fixes #131

[claude]

Code Review

Bug: Response body read after close in submit.go

Line 223: _ = outputResp.Body.Close() is called immediately after reading the body at line 222, but then the same body is read again in lines 231-235 for JSON unmarshaling. The body was already consumed and closed, making the second read fail.

Fix: move Close() to a defer after the status check:

if outputResp.StatusCode != 200 {
    defer outputResp.Body.Close()
    body, err := io.ReadAll(outputResp.Body)
    // ... rest of error handling
}

Duplication

Error parsing logic is duplicated in cat.go:103-115 and submit.go:221-236. Extract to a helper function to reduce repetition and ensure consistent error handling across commands.