Change "Exception" to "ValueError"

[colligant]

This PR changes the general "Exception" in api.py to ValueError.
From the python docs:

ValueError:
Raised when an operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError 

In general using "Exception" is a bit ambiguous and not recommended for production.

[FussyDuck]

All committers have signed the CLA.

[veenstrajelmer]

@EddyCMWF could you please review and merge this? this is a useful improvement.