Improve validation when reading WebVTT files

This is the first step in adressing ﻿OCTO-6084: ensuring that WebVTTReader catches most basic errors.

Once this is merged, library users can do something like this:

```python
# generate WebVTT
output = WebVTTWriter().write(input)

# validate generated WebVTT
WebVTTReader().read(output)  # will raise errors if the output is invalid  
```