Better error message

__tests__/diff-snapshot.spec.js

@@ -50,9 +50,30 @@ describe('diff-snapshot', () => {
       expect(mockSpawnSync).toBeCalled();
     });
 
-    it('throws when process returns a non-zero status', () => {
-      const runDiffImageToSnapshot = setupTest({ status: 1 });
-      expect(() => runDiffImageToSnapshot(fakeRequest)).toThrow();
+    it.each`
+      spawnReturn
+      ${{ status: 1 }}
+      ${{ status: 1, error: {} }}
+      ${{ status: 1, error: new Error() }}
+    `(
+  'throws an Unknown Error when process returns a non-zero status $#',
+  ({ spawnReturn }) => {
+    const runDiffImageToSnapshot = setupTest(spawnReturn);
+
+    expect(() => runDiffImageToSnapshot(fakeRequest)).toThrowError(
+      new Error('Error running image diff: Unknown Error')
+    );
+  }
+);
+
+    it('throws a helpful error if available', () => {
+      const runDiffImageToSnapshot = setupTest({
+        status: 1,
+        error: new Error('🦖'),
+      });
+      expect(() => runDiffImageToSnapshot(fakeRequest)).toThrowError(
+        new Error('Error running image diff: 🦖')
+      );
     });
   });
 

src/diff-snapshot.js

@@ -343,7 +343,7 @@ function runDiffImageToSnapshot(options) {
     const output = writeDiffProcess.output[3].toString();
     result = JSON.parse(output);
   } else {
-    throw new Error('Error running image diff.');
+    throw new Error(`Error running image diff: ${(writeDiffProcess.error && writeDiffProcess.error.message) || 'Unknown Error'}`);
   }
 
   return result;