Improve error handling

Summary

Currently, there's lot of code triggering a RuntimeError which in turn will make beat.editor stop.

Most of these situations should be gracefully handled. If possible, a fix should be proposed to the user so he can continue working.

Steps to reproduce

1. Create a new Plotter
2. Load the plotter

This will make beat.editor crash as the new Plotter will be based on the prototype that contains an invalid entry. This situation is already reported in #244 (closed)

What is the current bug behavior?

beat.editor will end there

What is the expected correct behavior?

beat.editor should not stop

Possible fixes

The user should get a message about what is going wrong. If possible a fix should be proposed and applied if accepted.

In any case, the editor should continue working.