Handle syntax without syntax or indent files

A syntax in the config without a syntax file, like postcss, showed
E484 on opening a file. Skip loading it, with a warning in debug.

A syntax without an indent file used cindent, which broke indent of
languages like pug, stylus without their plugins, or toml. Keep the
indent instead. Lines outside blocks still use cindent.

indent/vue.vim

@@ -75,6 +75,11 @@ function! s:GetBlockIndent(syntax)
   let indentexpr = get(s:indent, syntax) 
   if !empty(indentexpr)
     let ind = eval(indentexpr)
+  elseif has_key(s:indent, syntax)
+    " No indent file for syntax in config, keep indent as cindent may break
+    " languages like pug or toml
+    call vue#LogWithLnum('indentexpr not found for '.syntax.', keep indent')
+    let ind = -1
   else
     call vue#LogWithLnum('indentexpr not found for '.syntax.', use cindent')
     let ind = cindent(v:lnum)