fix(dita): report dita's own error output instead of just the exit status

[VXNCXNX]

lintDITA wires the dita command's stderr into a buffer and then never reads it, so a failed conversion shows only the exit status:

E100 [a.dita] Runtime error

exit status 1

The actual diagnostic — DTD resolution failures, malformed maps, missing conrefs — is discarded.

This returns the captured stderr, the way lintXML already does at internal/lint/xml.go:45:

E100 [a.dita] Runtime error

[DOTX046E][ERROR] The DTD could not be resolved.

Fixes #578

The fallback matters

Copying lintXML verbatim (errors.New(eut.String())) would regress the case where dita exits non-zero without writing anything — the error body would be blank, which is worse than today. So the exit status is kept as a fallback:

dita behaviour	before	after
writes to stderr, exits 1	exit status 1	the stderr text
exits 1 silently	exit status 1	exit status 1
succeeds	unchanged	unchanged

Verified both failing cases with a stub dita on PATH.

Worth being precise about the scope: dita writes some diagnostics to stdout rather than stderr, and those still will not appear. Only cmd.Stderr is captured here, and I did not want to conflate the two streams into one buffer since that would change what the message looks like in the common case.

go test ./internal/... passes. The DITA e2e cases are gated behind requires: dita and all cover the success path, which this does not touch.