[frontend] improve error messages that occur when required pipeline components are not included

[hmtosi]

When a pipeline is created in kale and a user hits Compile and Run without selecting necessary data, the extension throws RPC errors with complicated messages. In some of these scenarios, the solution is just to select the required component. It would be good for the error messages to be slightly cleaner in this case, so a user can figure out the fix more quickly.

The components covered in this ticket are pipeline task and pipeline name.

Steps to reproduce

1. Start with a new instance of kale (rebuild the extension and restart jupyterlab).
2. Open a new notebook and add a cell with a small function (ie. `print("hello world")
3. Compile and Run
4. This will produce an error saying "Not a valid Pipeline name: ''. Must match regex..."
5. Add a pipeline name
6. Compile and Run
7. This will produce an error saying "Task is missing from pipeline"
8. Click the pencil icon and add pipeline components to the hello world cell (select cell type and give it a name)
9. Compile and Run
10. Now it should run without error

[hmtosi]

Pipeline name may be fixed by #510