Add `errorformat` local option

[vm70]

Do you think it would be a good idea to add Mermaid CLI's multi-line error format to this filetype plugin?

Here's an example error from a simple Mermaid file I wrote:

graph LR
  a --> b
  c --> d
  e --> -->

Error: Parse error on line 4:
...b  c --> d  e --> -->
---------------------^
Expecting 'AMP', 'COLON', 'PIPE', 'TESTSTR', 'DOWN', 'DEFAULT', 'NUM', 'COMMA', 'NODE_STRING', 'BRKT', 'MINUS', 'MULT', 'UNICODE_TEXT', got 'LINK'

I think that corresponds to the following error format:

setlocal errorformat=%EError:\ Parse\ Error\ on\ line\ %l:
setlocal errorformat+=%C%.%#\ \ %s
setlocal errorformat+=%C%p^
setlocal errorformat+=%Z%m

In my fork of this repo, I condensed the above lines into one; but there are still some problems with how the column number is set.