distinguish error versus short read from read_in_full()

distinguish error versus short read from read_in_full()

Many callers of read_in_full() expect to see the exact
number of bytes requested, but their error handling lumps
together true read errors and short reads due to unexpected
EOF.

We can give more specific error messages by separating these
cases (showing errno when appropriate, and otherwise
describing the short read).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>