Throw a more correct exception for SEEK_END

[octylFractal]

In my opinion, throwing ValueError for seek(n, SEEK_END) on a stream_reader is incorrect. The Python io docs say (for the seek method as a whole):

Return True if the stream supports random access. If False, seek(), tell() and truncate() will raise OSError.

To me, this indicates that instead of throwing ValueError, seek() should throw an OSError. This would fix an issue with passing the stream_reader to requests' data parameter, which tries to read the length of a file-like object by issuing a SEEK_END, and catching OSError or IOError to indicate that it is unsupported. Source here.

[octylFractal]

Alternatively, I see the comment in the relevant code:

We /could/ support this with pos==0. But let's not do that until someone needs it.
This could be the time to add this support, as pos==0 is exactly how requests does this. I'm not particularly partial to one way or the other.

[browdus]

To add a bit of supporting evidence for this, we ran into this exact issue in boto/boto3#2555. This effectively means stream_reader can't be used directly with boto3's s3 upload_fileobj API because it attempts to probe for SEEK_END support, but gets an out-of-spec exception type.