Avoid printing unnecessary warnings during fetch and push

Avoid printing unnecessary warnings during fetch and push

If a transport doesn't support an option we already are telling
the higher level application (fetch or push) that the option is not
valid by sending back a >0 return value from transport_set_option
so there's not a strong motivation to have the function perform the
output itself.  Instead we should let the higher level application
do the output if it is necessary.  This avoids always telling the
user that depth isn't supported on HTTP urls even when they did
not pass a --depth option to git-fetch.

If the user passes an option and the option value is invalid we now
properly die in git-fetch instead of just spitting out a message
and running anyway.  This mimics prior behavior better where
incorrect/malformed options are not accepted by the process.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>