Abort setup on prefix-init failure, flag broken Proton installs

When stepProtonInit failed, the runner kept marching through every
downstream step against a half-initialized prefix, producing a cascade
of misleading "wine client error: version mismatch" failures that
masked the real problem. Seen in the wild with a GE-Proton10-34 install
that was missing concrt140.dll from its bundled default_pfx template:
setup_prefix crashes with FileNotFoundError, then seven unrelated steps
are reported as broken.

Break out of the step loop as soon as proton_init fails, and inspect the
captured Proton output for the "FileNotFoundError" + "default_pfx" hint
so the error message points at the broken Proton distribution instead
of a generic "exit code 1". runProcess gains an optional QByteArray*
out param so the caller can pattern-match on the merged output without
changing any existing call sites.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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