wayland-egl: Make symbol test fail on failure

The previous rewrite of the wayland-egl ABI checker introduced checks
for removed symbols as well as added symbols, but broke some failure
conditions. Add an explict return-code variable set in failure paths,
rather than chaining or conditions.

If we cannot find the binary or nm, we regard this as an error
condition, rather than test failure.

v2: Don't test if we can execute $NM.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Fixes: 21b1f22e ("wayland-egl: enhance the symbol test")
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>