Fix incorrect ValueError for user/token not found

[joente]

Description

This pull request fixes an incorrect error when using a function where a user or token is not found. Both a token and user do not honor the naming rules but when not found, the error message is generated by a function which is build for names which must apply to the naming rule.

Example:

del_user('123');  // user 123 does not exist

The code above returns with a ValueError (naming rule) but 123 is valid for a user.

Expected:
A LookupError is expected (the same applies for a token not found).

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Test thingsdb functions

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes