Add detail in error messages when yarn plugin can't detect backstage version

I was confused for ~15 minutes today when updating to using the yarn
plugin. It was failing in docker but not locally, and I didn't know why.
It turned out to be because I forgot to copy the backstage.json into the
docker image. This was confusing, because the error seemed to indicate I
was failing the semver checks. This change propagates error detail down
the line, so people will see the actual cause. (i.e. missing file, no
version field, semver wrong)

Signed-off-by: Riley Martine <rmartine@integralads.com>

yarn.lock

@@ -49826,6 +49826,7 @@ __metadata:
     "@backstage/backend-test-utils": "workspace:^"
     "@backstage/cli": "workspace:^"
     "@backstage/cli-common": "workspace:^"
+    "@backstage/errors": "workspace:^"
     "@backstage/release-manifests": "workspace:^"
     "@yarnpkg/builder": "npm:^4.2.1"
     "@yarnpkg/core": "npm:^4.4.1"