[Lens] change copy for error messages and normalize by unit tooltip

[mbondyra]

Tooltip for normalize by unit, change content to:

When "second" is selected: Display values as rate per second instead of using the underlying date interval.
When "minute" is selected: Display values as rate per minute instead of using the underlying date interval.

Error messages

1. Error message on the workspace panel

Instead:

2 invalid references

(depends on the error type)
The field [field name] cannot be found in the index. Use a different field.
The field [field name] has changed type and can no longer be used.

2. Error message on the dimension editor

- align with the workspaces messages

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[stratoula]

I think this has been addressed in recent kibana versions