library panics when input contains unkown admontion type

[getreu]

Consider the following rst input:

hello

.. DANGER::
   Beware killer rabbits!

My thread panics because rust-rst panics.

thread '<unnamed>' panicked at 'Unknown admontion type DANGER!', /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/rst_parser-0.4.0/src/conversion/block.rs:189:24

Expected behaviour

Either silently ignore unknown admontions or return an error type.

[getreu]

I also experienced some panics on some not implemented language features. A parser library should never panic, but return an error instead.