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>

packages/yarn-plugin/package.json

@@ -31,6 +31,7 @@
   },
   "dependencies": {
     "@backstage/cli-common": "workspace:^",
+    "@backstage/errors": "workspace:^",
     "@backstage/release-manifests": "workspace:^",
     "@yarnpkg/core": "^4.4.1",
     "@yarnpkg/fslib": "^3.1.2",

packages/yarn-plugin/src/util/getCurrentBackstageVersion.test.ts

@@ -73,17 +73,15 @@ describe('getCurrentBackstageVersion', () => {
   });
 
   it.each`
-    description              | content
-    ${'is missing'}          | ${{}}
-    ${'is invalid'}          | ${{ 'backstage.json': '}{' }}
-    ${'has missing version'} | ${{ 'backstage.json': '{"a":"b"}' }}
-    ${'has invalid version'} | ${{ 'backstage.json': '{"version":"foobar"}' }}
-  `('throws if backstage.json $description', ({ content }) => {
+    description              | content                                         | message
+    ${'is missing'}          | ${{}}                                           | ${/valid version string not found.*no such file/i}
+    ${'is invalid'}          | ${{ 'backstage.json': '}{' }}                   | ${/valid version string not found.*not valid json/i}
+    ${'has missing version'} | ${{ 'backstage.json': '{"a":"b"}' }}            | ${/valid version string not found.*version field is missing/i}
+    ${'has invalid version'} | ${{ 'backstage.json': '{"version":"foobar"}' }} | ${/valid version string not found.*exists but is not valid semver/i}
+  `('throws if backstage.json $description', ({ content, message }) => {
     mockDir.addContent(content);
 
-    expect(() => getCurrentBackstageVersion()).toThrow(
-      /valid version string not found/i,
-    );
+    expect(() => getCurrentBackstageVersion()).toThrow(message);
   });
 
   it('caches repeated calls', () => {

packages/yarn-plugin/src/util/getCurrentBackstageVersion.ts

@@ -18,6 +18,7 @@ import assert from 'assert';
 import { valid as semverValid } from 'semver';
 import { ppath, xfs } from '@yarnpkg/fslib';
 import { BACKSTAGE_JSON } from '@backstage/cli-common';
+import { ForwardedError } from '@backstage/errors';
 import { memoize } from './memoize';
 import { getWorkspaceRoot } from './getWorkspaceRoot';
 
@@ -26,11 +27,16 @@ export const getCurrentBackstageVersion = memoize(() => {
 
   let backstageVersion: string | null = null;
   try {
-    backstageVersion = semverValid(xfs.readJsonSync(backstageJsonPath).version);
+    const backstageVersionRaw = xfs.readJsonSync(backstageJsonPath).version;
+    assert(backstageVersionRaw !== undefined, 'Version field is missing');
+    backstageVersion = semverValid(backstageVersionRaw);
 
-    assert(backstageVersion !== null);
-  } catch {
-    throw new Error('Valid version string not found in backstage.json');
+    assert(backstageVersion !== null, 'Version exists but is not valid semver');
+  } catch (err) {
+    throw new ForwardedError(
+      'Valid version string not found in backstage.json',
+      err,
+    );
   }
 
   return backstageVersion;