MessageFormatter does not perform the full parsing at configuration time

[stof]

When encountering nested tokens, they are parsed only during formatting, not when compiling the pattern.
And the type of tokens are validated only during the rendering as well. But the native implementation detects such error during the instantiation: https://3v4l.org/Sf69D
This means that the error handling required for the polyfill is not the same than for the native implementation.