Fix problems with nim check (#404)

* Fix problems with nim check

- Multi file errors are not shown in the same buffer
- Fixes parsing of error type that contain ':'

* Remove redundant fnameescape

test/handler/test_nim_handler.vader

@@ -24,10 +24,18 @@ Execute(Parsing nim errors should work):
   \     'text': 'Hint: ''NotUsed'' is declared but not used [XDeclaredButNotUsed]',
   \     'type': 'W',
   \   },
+  \   {
+  \     'bufnr': 42,
+  \     'lnum': 12,
+  \     'col': 2,
+  \     'text': 'Error: with : character',
+  \     'type': 'E',
+  \   },
   \ ],
   \ ale_linters#nim#nimcheck#Handle(42, [
   \ 'Line with wrong( format)',
   \ 'foobar.nim(8, 8) Warning: use {.base.} for base methods; baseless methods are deprecated [UseBase]',
   \ 'foobar.nim(12, 2) Error: identifier expected, but found ''a.barfoo''',
   \ '/nested/folder/foobar.nim(2, 5) Hint: ''NotUsed'' is declared but not used [XDeclaredButNotUsed]',
+  \ 'foobar.nim(12, 2) Error: with : character',
   \ ])