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.

test/run.vim

@@ -92,7 +92,8 @@ function! ExpectTag(lnum, tag)
 endfunction
 
 " Call GetVueIndent() for each non-blank line from line 2 as when typing it,
-" which shows errors hidden by 'indentexpr', and check the indent
+" which shows errors hidden by 'indentexpr', and check the indent. -1 keeps
+" the indent
 function! ExpectIndentEachLine()
   for lnum in range(2, line('$'))
     if getline(lnum) =~ '^\s*$'
@@ -101,7 +102,8 @@ function! ExpectIndentEachLine()
     call cursor(lnum, 1)
     let v:lnum = lnum
     try
-      call ExpectEqual(GetVueIndent(), indent(lnum),
+      let ind = GetVueIndent()
+      call ExpectEqual(ind == -1 ? indent(lnum) : ind, indent(lnum),
             \ printf('GetVueIndent() at line %d', lnum))
     catch
       call Fail(printf('GetVueIndent() at line %d: %s', lnum, v:exception))