ENH: Add AssertError exception string to output tool validation

[liukidar]

Problem
The tool validation mechanism is quite consistent in raising a clear self-explainatory message when a condition is not met, except for:

# Validate output type
assert getattr(self, "output_type", None) in AUTHORIZED_TYPES

For which a message is not provided.

Proposed solution
Add an error message to the assertion, such as

# Validate output type
output_type = getattr(self, "output_type", None)
assert output_type in AUTHORIZED_TYPES, f"Output type '{output_type}': type must be one of the following values: {AUTHORIZED_TYPES}."

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Checklist

• I have created a pull request #1861.