[RDY] vim-patch:7.4.327

Problem:    When 'verbose' is set to display the return value of a function, may get E724 repeatedly.
Solution:   Do not give an error for verbose messages. Abort conversion to string after an error.

https://code.google.com/p/vim/source/detail?r=99d8f2d72dcd4b850de81998cc9b1120c8165762

Changes to original patch:
* `did_echo_string_emsg` is now of type `bool` instead of 'int`
* comments according to Neovim style guide