Braille conversions produce more warnings than needed

There are some warnings that the user can not do anything about. Some examples:

- Liblouis may try to compile a file that is not a Liblouis table. This generates a warning but should at most be a debug message. It is not relevant for the user and definitely not as a warning.
- A certain combination of CSS rules may justify a warning (to help the user detect mistakes), but if the user is not in full control of the style sheet, such warnings are sometimes unavoidable and can be confusing.
- https://github.com/daisy/pipeline-modules/issues/159

Sometimes we can defer warnings to when they really matter:

- For instance, showing a warning such as `text-transform: uncontracted` not being supported is only noise when the property is never used.