Allow local testing when referencing lambdas in schema that exist outside project

**Is your feature request related to a problem? Please describe.**
I would like to submit a pull request to change this line in `packages/amplify-util-mock/src/api/api.ts`:

```js
throw new Error(
'Local mocking does not support AWS_LAMBDA data source that is not provisioned in the project.\nEnsure that the environment is specified as described in https://aws-amplify.github.io/docs/cli-toolchain/graphql#function',
);
```
To be a warning. I would still like to test other functions that are part of the amplify project locally.

**Describe the solution you'd like**
Just warn the user instead of throwing an error.

**Describe alternatives you've considered**
commenting out the code that references functions that are not part of the project, but then the risk is committing it to the repo accidentally.