Improve error messages when loading workflows

Attempting to load an updated workflow file with the same version number causes a unique constraint violation. The workflow fails to load. The reason can be seen in the stacktrace on the server:

```
IntegrityError: UNIQUE constraint failed: data_workflowversion.workflow_id, data_workflowversion.version
```

But the response to the client is simply:

```
{"detail":"Unable to import workflow"}
```