Return EIO on failed read

Previously S3fsMultiCurl::MultiRead did not report read errors since
it did not treat failed callback setup as a fatal operation error.
Failed callback setups usually result from exceeding the number of
allowed retries.  Previously cp did not report an error during a
network outage but now does:

$ cp ~/s3-path/s3-file .
cp: error reading ‘/home/gaul/s3-path/s3-file’: Input/output error
cp: failed to extend ‘./s3-file’: Input/output error
Merge pull request #64 from andrewgaul/failed-read-eio

Return EIO on failed read