Do not set File._errno when reading less than requested bytes.

[mfelsche]

While investigating a potential bug in my implementation of #2707 I found this one:

When requesting more bytes than available from a File the _errno was set to
although reading less than available bytes (> 0) is no error condition.
If anything then FileEOF should be set in this case to signal that we reached the end of file.
Nonetheless i kept it as it is right now, as i think it is more reliable to detect FileEOF when reading 0 bytes.

[jemc]

Hmmm.. not sure about this, due to this detail from the manual for read:

POSIX allows a read() that is interrupted after reading some data to return -1 (with errno set to EINTR) or to return the number of bytes already read.

[mfelsche]

@jemc i read that as, either it returns -1, which is fine, even with the change in this PR, errno will be fetched and set on File._errno. The other case should also be fine. If it was interrupted but did read some data, it will return the length and the data will be there in the buffer. Do you see this as an error condition we should handle differently?

[jemc]

Hmm... okay, sounds good.