errorformat compatibility issues

[danielnehrig]

https://github.com/HerringtonDarkholme/yats.vim/blob/master/compiler/typescript.vim#L21

i'd like suggest the change to += adding the formatting to the error format since overwriting the error format will remove compatibility with different plugins that add a match to the errorformat

example: using vim-test / neomake with jest for testing in vim and using yats typescript tsc compile both have a own errorformat for the filetype typescript and instead of overwritting the efm i'd suggest appending