Suppress unmap errors at undo_ftplugin

[grauwoelfchen]

This pull request suppresses errors relate to unmappings at unloading of this plugin.

The nmap/vmap both are currently set only if those mappings aren't set yet by others (with chesk of maparg(...) at L44 and L63). So, if these buffer local mappings are not set (I had another mapping for K), then errors like below occur when b:undo_ftplugin is evaluated, since they don't exist.

Error detected while processing BufRead Autocommands for "*.rkt"..function <SNR>55_RacketDetectHashLang[3]..FileType Autocommands 
for "*"..function <SNR>20_LoadFTPlugin:
line    3:
E31: No such mapping

I suppose these errors would be harmless (because buffer local mappings does not exist), but a bit annoying, so I've simply added silent! at here 😄

[grauwoelfchen]

I found this during an investigation of another issue at purescript-contrib/purescript-vim#91, because this unloading part seems to be unexpectedly? evaluated due to that when I just opened a Racket file.

[benknoble]

Something similar is already achieved in https://github.com/benknoble/vim-racket/blob/1c02b65f30d01eb05049e0a3f1d7ddb84658457c/ftplugin/racket.vim#L64 which ships with latest Vim. I don't remember why execute is necessary but I'm fairly confident it is (possibly to do with separating from the commands that come after).

[grauwoelfchen]

That's nice!

Speaking of execute, I also had an experiment unmapping them conditionally with if like below. Maybe that's a bit wordy, but it somehow worked even without silent!. This version didn't work without using those exucutes 🤔

let b:undo_ftplugin =
      \  "setl iskeyword< lispwords< lisp< comments< formatoptions<"
      \. "| setl makeprg< commentstring<"
      \. "| if maparg('K', 'n') ==# '<Plug>RacketDoc' \| execute 'nunmap <buffer> K' \| endif"
      \. "| if maparg('K', 'v') ==# '<Plug>RacketDoc' \| execute 'vunmap <buffer> K' \| endif"

Anyway, I agree with that. That would fairly be better to have it in a string for clear separation.
So I'll add executes here too. Thanks 🙋🏽‍♀️