submodule: prefix die messages with 'fatal'

submodule: prefix die messages with 'fatal'

The standard `die()` function that is used in C code prefixes all the
messages passed to it with 'fatal: '. This does not happen with the
`die` used in 'git-submodule.sh'.

Let's prefix each of the shell die messages with 'fatal: ' so that when
they are converted to C code, the error messages stay the same as before
the conversion.

Note that the shell version of `die` exits with error code 1, while the
C version exits with error code 128. In practice, this does not change
any behaviour, as no functionality in 'submodule add' and 'submodule
update' relies on the value of the exit code.

Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>