Improve validation error messages for better user experience

[bergr7]

Description

The current validation error messages in the flow-merge validate --config ... tool could be improved to provide a better user experience. When a user encounters a validation error, the message should clearly explain what went wrong and provide actionable steps to resolve the issue.

For example, the current error message for a missing required field in the configuration file looks like this:

Configuration file is invalid: 1 validation error for ValidatedInputData
models.0.model
  Field required [type=missing, input_value={'models': 'TheBloke/Llama-2-13B-fp16'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.4/v/missing

An improved error message could start with:

Error: Missing required field '<name_of_field>' in the configuration file.

...

Example of a correct model entry:
models:
  - model: TheBloke/Llama-2-13B-fp16
    weight:
    
...