lualoader: Fix try_include error handling

The previous iteration of try_include attempted to be 'friendly' and error()
out if we hit an error that wasn't ENOENT. This was semi-OK, but fragile as
it relied on pattern matching the error message.

Move the responsibility for handling failure to the caller. Following
a common lua pattern, we'll return the return value of the underlying
require() on success, or false and an error message.

Reported by:	bcran
MFC after:	3 days

stand/lua/core.lua

@@ -66,23 +66,15 @@ end
 
 
 -- Globals
--- try_include will return the loaded module on success, or nil on failure.
+-- try_include will return the loaded module on success, or false and the error
--- A message will also be printed on failure, with one exception: non-verbose
+-- message on failure.
--- loading will suppress 'module not found' errors.
 function try_include(module)
 	local status, ret = pcall(require, module)
 	-- ret is the module if we succeeded.
 	if status then
 		return ret
 	end
-	-- Otherwise, ret is just a message; filter out ENOENT unless we're
+	return false, ret
-	-- doing a verbose load. As a consequence, try_include prior to loading
-	-- configuration will not display 'module not found'. All other errors
-	-- in loading will be printed.
-	if config.verbose or ret:match("^module .+ not found") == nil then
-		error(ret, 2)
-	end
-	return nil
 end
 
 -- Module exports