trace: correct variable name in write() error message

trace: correct variable name in write() error message

Our error message for write() always mentions GIT_TRACE,
even though we may be writing for a different variable
entirely. It's also not quite accurate to say "fd given by
GIT_TRACE environment variable", as we may hit this error
based on a filename the user put in the variable (we do
complain and switch to stderr if the file cannot be opened,
but it's still possible to hit a write() error on the
descriptor later).

So let's fix those things, and switch to our more usual
"unable to do X: Y" format for the error.

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