bitbake: fetch2: npm: conditionally hide NPM_LOCKDOWN / NPM_SHRINKWRAP warnings

bitbake: fetch2: npm: conditionally hide NPM_LOCKDOWN / NPM_SHRINKWRAP warnings

If ud.ignore_checksums is set (which we currently use to suppress the
warnings for missing SRC_URI checksums when fetching files from
scripts), then if we're fetching an npm package we should similarly
suppress the warnings when NPM_LOCKDOWN and NPM_SHRINKWRAP aren't set.

At the same time, make any errors reading either of these files actual
errors since if the file is specified and could not be found, that
should be an error - not the exact same warning.

Fixes [YOCTO #10464].

(Bitbake rev: cefb8c93c8299e68352cf7ec5ad9ca50c0d499ed)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>