fixes related to embedded syntax for code blocks

There are 2 different fixes here:

1. Embedded syntax highlights did not work when a code block had an
   offset due to missing check for possible spaces in the "end" pattern
   for region "pandocDelimitedCodeBlock_" . s:langname.
2. Pandoc allows complex language preambles like
     ~~~~ {.cpp hl="vim" vars="PhHtmlEngine=tohtml,load_TagHl"}
   (an example taken from http://github.com/lyokha/vim-publish-helper).
   In this case if list g:pandoc_use_embeds_in_codeblocks_for_langs
   contains "vim" this will erroneously trigger highlights for vim
   though the block contains code from unlisted "cpp". This commit fixes
   this according to pandoc's rules for setting languages in code
   blocks.

amended by blaenk to remove the third change
be silent if specific syntax file is missing