uploader: propagate errors in compress

[teqwve]

Hi,

we've noticed that compress goroutine does not properly handle corner cases: it does not close upstream and it does not propagate an error in case it occurs. Because of that with compression enabled uploader behaved differently than with compression disabled (when there intermediary pipes). This PR adds a proper handling.

[teqwve]

Maybe I'll explain why I believe this fix is important - it may cause strange bugs, in our case it caused a worker goroutine to be blocked in a file upload. Blocked goroutines weren't releasing files (so files were never returned to the pool and re-uploaded) and once number of blocked goroutines reached configured number of upload 'threads' the upload was blocked completely.

In this case it was triggered by similar behavior as described in https://github.com/go-graphite/carbon-clickhouse/pull/138:

• ClickHouse returns 200 ok without reading the whole request, before carbon clickhouse has sent the whole file
• go http client closes the read end of body pipe
• io.Copy in compress goroutine fails with ErrClosedPipe and exits without propagating the error upstream (here)
• parseAndFilter in worker goroutine is blocked forever on writing to a pipe nobody is going to read from (here)

it wasn't happening with compression disabled, so I think that we should fix error handling in compression (in a similar way it's handled in compress goroutine example in go sources, https://github.com/golang/go/blob/201a2e9c2f82dd2c57c8e79bbe2c028d7c13b8ea/src/compress/gzip/example_test.go#L169).

I may also add that this seem to be a quite common issue, e.g. an example I've found in gist have it, https://gist.github.com/tomcatzh/cf8040820962e0f8c04700eb3b2f26be