exception documentation
Exception raised from .unwrap_<...> and .expect_<...> calls.
The original Result
can be accessed via the .result attribute, but
this is not intended for regular use, as type information is lost:
UnwrapError doesn't know about both T_co and E_co, since it's raised
from Ok() or Err() which only knows about either T_co or E_co,
not both.