Report socket failures as reasons, not as MatchError messages

[gmile]

URP.Protocol wrapped :gen_tcp.send and :gen_tcp.recv in a match on success, so a socket that timed out or closed mid-call raised a MatchError. Every layer above rescues and stringifies, so the only account a caller got of a timed-out conversion was this:

{:error, "no match of right hand side value:\n\n    {:error, :timeout}\n"}

Two changes:

URP.Protocol now raises URP.SocketError, which carries the reason :gen_tcp reported.

URP.convert/2 and friends return that reason as an atom — :timeout, :closed, a POSIX error — while anything soffice itself said stays a message string. That is the distinction a caller actually needs: retrying a document soffice refused is pointless, and a wedged soffice is not the document's fault. The bang functions re-raise a socket reason as the URP.SocketError it was, so they still raise something with a readable message.

Breaking for callers that assumed every reason was a binary, so this wants a minor bump. VERSION and the changelog heading are left alone — release.sh minor cuts those.

Tests cover both reasons over a real socket pair, a body that stops arriving mid-frame, a send to a dead socket, the message wording, and an atom reason passing through checkout_outcome/4. mix test, mix format --check-formatted, mix dialyzer, mix xref graph --format cycles and mix docs --warnings-as-errors all pass locally; the integration suite against a real soffice has not been run here.