First off, avoid potential circular references (via {qx_arg}) by
dropping the {-qsp} field from $ctx and SolverGit objects.
Instead, we only share a reference to an optional error buffer
string {qsp_err}.
We'll also attempt to call qspawn.wcb if qx_cb fails, and warn
in more places w/o checking for $env since we now rely on warn()
instead of $env->{'psgi.errors'}.
This makes error handling simpler and safer in future callers.