Make GitHub import errors more descriptive (codesandbox#3430)

* Make GitHub import (and other?) errors more descriptive.

* Check if GitHub import.

Author: lbogdan

packages/app/src/app/overmind/namespaces/editor/actions.ts

@@ -100,7 +100,11 @@ export const sandboxChanged: AsyncAction<{ id: string }> = withLoadApp<{
     actions.workspace.openDefaultItem();
   } catch (error) {
     state.editor.notFound = true;
-    state.editor.error = error.message;
+    let detail = error.response?.data?.errors?.detail;
+    if (Array.isArray(detail)) {
+      detail = detail[0];
+    }
+    state.editor.error = detail || error.message;
     state.editor.isLoading = false;
     return;
   }

packages/app/src/app/pages/Sandbox/index.tsx

@@ -77,6 +77,7 @@ export const Sandbox: React.FC<Props> = ({ match }) => {
             Something went wrong
           </div>
           <Title style={{ fontSize: '1.25rem', marginBottom: 0 }}>
+            {isGithub ? 'Error importing GitHub repository: ' : ''}
             {state.editor.error}
           </Title>
           <br />