Invalid expression in generated vim script

[tyru]

Invalid expression should occur when lazy loading is failed.

volt/plugconf/plugconf.go

Lines 323 to 328 in 8f2139f

	if exists(':' . a:command) isnot# 2
	echohl ErrorMsg
	echomsg printf('[volt] Lazy loading of Ex command '%s' failed: '%s' is not found', a:command, a:command)
	echohl None
	return
	endif

The single quotes around %s are in another single quotes.
So that should be escaped like ''%s''.