add tests for lightline#error()

test/error.vim

@@ -0,0 +1,15 @@
+let s:suite = themis#suite('error')
+let s:assert = themis#helper('assert')
+
+function! s:message() abort
+  redir => messages
+    silent! messages
+  redir END
+  return split(messages, '\n')[-1]
+endfunction
+
+function! s:suite.error()
+  let message = 'An error occurred.'
+  call lightline#error(message)
+  call s:assert.equals(s:message(), 'lightline.vim: ' . message)
+endfunction