Potentially confusing print by YAML checker when checking a loader is first in the pipeline

There's a check that is making sure that the first method in the pipeline YAML file is a loader:https://github.com/DiamondLightSource/httomo/blob/2e15a80e8e96753bc8a0d13285209cdbd9a56434/httomo/yaml_checker.py#L102-L122

If the check passes, the message printed is: https://github.com/DiamondLightSource/httomo/blob/2e15a80e8e96753bc8a0d13285209cdbd9a56434/httomo/yaml_checker.py#L120

However, in the event that:
- this check passes
- but other checks fail in the loader (such as a duplicate parameter)

this message being printed is confusing, because "loader check successful" implies that the loader config is error-free, but in reality this is not the case.

I think the message should be updated to be more specific in stating that the check for the first method being a loader was successful, to avoid such confusion in cases where the check passes but other checks that involve the loader fail. Maybe something like:
```
Loader first in pipeline check successful
```