Add error handling to VimwikiSearch per issue #420

- Create function wrapper around lvimgrep for input checking, pattern
   quoting, and error handling.
 - Add Vader tests for VimwikiSearch.
 - Change syntax loading from try/catch to explicit file check (to
   prevent Vader test bug).
 - Update doc/vimwiki.txt for changes.
 - Change test script to run Vader tests separately

test/run_tests.sh

@@ -35,9 +35,11 @@ runVader() {
     for v in $vers; do
         echo ""
         echo "Running version: $v"
+        vim="/vim-build/bin/$v -u test/vimrc -i NONE"
+        test_cmd="for VF in test/*.vader; do $vim \"+Vader! \$VF\"; done"
         set -o pipefail
         docker run -a stderr -e VADER_OUTPUT_FILE=/dev/stderr "${flags[@]}" \
-            "$v" -u test/vimrc -i NONE "+Vader! test/*" 2>&1 | vader_filter | vader_color
+          /bin/bash -c "$test_cmd" 2>&1 | vader_filter | vader_color
         set +o pipefail
     done
 }