operations: fix ignored error in rcat probe reads

Return the first non-EOF input error encountered while Rcat probes whether an
upload is small. Previously that error was ignored and the full probe buffer,
including bytes that were never read, could be passed to Put or PutStream.

(cherry picked from commit b93b73bcb0)
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f