wget -nc + file already exists: fixed inconsistent exit codes

[Leedus McClein]

Exit code should exclusively be 0 or 1, and not 1 if -O is used and 0 if -O
isn't used. Code fixed here:
https://github.com/ProximaNova/gnu-wget

Which says:
"
With the mainsteam Wget, if the file already exists:
* WGET_EXIT_SUCCESS (exit code 0) from wget -nc https://example.com/
* WGET_EXIT_GENERIC_ERROR (exit code 1) from wget -nc https://example.com/
-O index.html

With this edit to Wget, if the file already exists:
* WGET_EXIT_GENERIC_ERROR (exit code 1) from wget -nc https://example.com/
* WGET_EXIT_GENERIC_ERROR (exit code 1) from wget -nc https://example.com/
-O index.html
"

Related:
-
https://lists.gnu.org/archive/cgi-bin/namazu.cgi?query=clobber+exit+code&submit=Search%21&idxname=bug-wget&max=20&result=normal&sort=score
- https://lists.gnu.org/archive/html/bug-wget/2018-07/msg00036.html