Fail silently when syntax has no lexer

This'll patch vis.lexers.load to return nil when the lexer could not be
found. Previously it would've errored out, which the load in lexer.lua
still will as this is used in lexers themselves.

Another possibility is to only patch set_syntax in vis.lua and the
WIN_HIGHLIGHT handler in vis-std.lua, but as most references to
vis.lexers.load already handle a nil return, this seems better.

lua/plugins/filetype.lua

@@ -506,15 +506,7 @@ vis.events.subscribe(vis.events.WIN_OPEN, function(win)
 		for _, cmd in pairs(filetype.cmd or {}) do
 			vis:command(cmd)
 		end
-		if not vis.lexers.property then return end
-		local path = vis.lexers.property['scintillua.lexers']:gsub(';', '/?.lua;')
-		local lexname = filetype.alt_name or syntax
-		local lexpath = package.searchpath(lexname, path)
-		if lexpath ~= nil then
-			win:set_syntax(lexname)
-		else
-			win:set_syntax(nil)
-		end
+		win:set_syntax(filetype.alt_name or syntax)
 	end
 
 	local path = win.file.name -- filepath

lua/plugins/textobject-lexer.lua

@@ -8,13 +8,17 @@ vis:textobject_new("ii", function(win, pos)
 		return nil
 	end
 
+	local lexer = vis.lexers.load(win.syntax, nil, true)
+	if not lexer then
+		return nil
+	end
+
 	local before, after = pos - MAX_CONTEXT, pos + MAX_CONTEXT
 	if before < 0 then
 		before = 0
 	end
 	-- TODO make sure we start at a line boundary?
 
-	local lexer = vis.lexers.load(win.syntax, nil, true)
 	local data = win.file:content(before, after - before)
 	local tokens = lexer:lex(data)
 	local cur = before

lua/vis.lua

@@ -131,7 +131,8 @@ else
 	local load_lexer = vis.lexers.load
 	vis.lexers.load = function (name, alt_name, cache)
 		if cache and lexers[alt_name or name] then return lexers[alt_name or name] end
-		local lexer = load_lexer(name, alt_name)
+		local status, lexer = pcall(load_lexer, name, alt_name)
+		if not status then return nil end
 		if cache then lexers[alt_name or name] = lexer end
 		return lexer
 	end
@@ -276,6 +277,7 @@ vis.types.window.set_syntax = function(win, syntax)
 		win.syntax = nil
 		return true
 	end
+	win.syntax = syntax
 
 	if not lexers.load then return false end
 	local lexer = lexers.load(syntax)
@@ -297,7 +299,6 @@ vis.types.window.set_syntax = function(win, syntax)
 		if style ~= nil then win:style_define(id, style) end
 	end
 
-	win.syntax = syntax
 	return true
 end