Add the exception assertion mechanism.

+ Add vimtest#assert#exception#new().
+ Add the exception assertion object.
+ Remove "current_expecteed_throw" member from the assertion object.
+ Remove vimtest#util#get_error_id().
Add throw_match() assertion method to the assertion object.
Fixed typo on assert.throw_match() document.
Merge pull request #9 from AmaiSaeta/feature/assert-throw_match/throw_match

Add throw_match() assertion method