more strict error type checking in most packages and backend plugins

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/e2e-test/package.json

@@ -25,6 +25,7 @@
   },
   "devDependencies": {
     "@backstage/cli-common": "^0.1.1",
+    "@backstage/errors": "^0.1.2",
     "@types/fs-extra": "^9.0.1",
     "@types/node": "^14.14.32",
     "chalk": "^4.0.0",

packages/e2e-test/src/lib/helpers.ts

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import { assertError } from '@backstage/errors';
 import {
   spawn,
   execFile as execFileCb,
@@ -66,11 +67,12 @@ export async function runPlain(cmd: string[], options?: SpawnOptions) {
     });
     return stdout.trim();
   } catch (error) {
+    assertError(error);
     if (error.stdout) {
-      process.stdout.write(error.stdout);
+      process.stdout.write(error.stdout as Buffer);
     }
     if (error.stderr) {
-      process.stderr.write(error.stderr);
+      process.stderr.write(error.stderr as Buffer);
     }
     throw error;
   }
@@ -147,6 +149,7 @@ export async function waitForPageWithText(
       );
       break;
     } catch (error) {
+      assertError(error);
       if (error.message.match(EXPECTED_LOAD_ERRORS)) {
         loadAttempts++;
         if (loadAttempts >= maxLoadAttempts) {