[dagit] Don't block viewing PythonError on Playground preview

Summary:
When an error occurs in generating the run preview pane on the Dagit playground, we have a PythonError but aren't correctly clearing the loading overlay. Fix this by only showing the overlay when the preview is actually loading. I'm not sure if I introduced this bug recently, but hopefully I'm not overlooking anything here.

Also change the error link to be a proper ButtonLink.

Test Plan: Yield a `SerializableErrorInfo` from `GrpcServerRepositoryLocation.get_subset_external_pipeline_result`, open playground and select a solid. Verify that the error is clickable and not blocked by the loading overlay.

Reviewers: dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D6435

js_modules/dagit/src/execute/ExecutionSessionContainer.tsx

@@ -600,7 +600,7 @@ const ExecutionSessionContainer: React.FC<IExecutionSessionContainerProps> = (pr
       second={
         <>
           <LoadingOverlay
-            isLoading={!runConfigSchema || previewLoading}
+            isLoading={previewLoading}
             message={!runConfigSchema ? LOADING_CONFIG_SCHEMA : LOADING_RUN_PREVIEW}
           />
           <RunPreview

js_modules/dagit/src/execute/RunPreview.tsx

@@ -280,9 +280,10 @@ export class RunPreview extends React.Component<RunPreviewProps, RunPreviewState
       errorsAndPaths.push({
         pathKey: '',
         error: (
-          <>
-            PythonError <span onClick={() => showCustomAlert({body: info})}>click for details</span>
-          </>
+          <span>
+            PythonError:{' '}
+            <ButtonLink onClick={() => showCustomAlert({body: info})}>Click for details</ButtonLink>
+          </span>
         ),
       });
     }