util/hybrj: invalid xtol currently hard-aborts instead of returning solver status

[zingale]

Summary

For invalid xtol (xtol < 0), hybrj calls amrex::Error, which aborts execution. This differs from MINPACK-style status reporting and makes the solver harder to integrate in robust workflows.

Affected code

• util/hybrj/hybrj.H: input validation path for xtol

Why this is a problem

A hard abort bypasses normal solver error handling and can terminate larger applications unexpectedly.

Expected behavior

The solver should return a non-success info code (e.g., improper input) and exit cleanly without process termination.

Suggested fix

• Replace hard abort with status-based return (info = 0 / dedicated bad-input code).
• Keep behavior consistent with other termination paths that set info and return.

Acceptance criteria

• Invalid xtol does not abort process.
• Caller can observe and handle the returned status code.