Fix the following error reported by Coverity:
```
Defect type: SUPPRESSED_ERROR
1. osbuild-composer-75/_build/src/github.com/osbuild/osbuild-composer/internal/manifest/os.go:546: suppressed_error: The error returned by the function "GetVerStrFromPackageSpecList(p.packageSpecs, "dracut-config-rescue")" is not explicitly checked.
2. osbuild-composer-75/_build/src/github.com/osbuild/osbuild-composer/internal/manifest/os.go:546: remediation: Explicitly check the return error.
544| }
545|
546|-> rescueVer, _ := rpmmd.GetVerStrFromPackageSpecList(p.packageSpecs, "dracut-config-rescue")
547| hasRescue := rescueVer != ""
548| bootloader = osbuild.NewGrub2LegacyStage(
```
Change the code to check if the called function returned an error or
not. If not error is returned, then the function returns a package
version string.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
f8b39cd34e