file: return errors for use in assertions

file.lua

@@ -25,16 +25,16 @@ end
 -- concat_path is set by init.lua to avoid code duplication
 
 function read(filename)
-	local file = io.open(filename, "r")
-	if file == nil then return nil end
+	local file, err = io.open(filename, "r")
+	if file == nil then return nil, err end
 	local content = file:read"*a"
 	file:close()
 	return content
 end
 
 function write_unsafe(filename, new_content)
-	local file = io.open(filename, "w")
-	if file == nil then return false end
+	local file, err = io.open(filename, "w")
+	if file == nil then return false, err end
 	file:write(new_content)
 	file:close()
 	return true
@@ -42,9 +42,9 @@ end
 
 write = minetest and minetest.safe_file_write or write_unsafe
 
-function write_binary(filename, new_content)
-	local file = io.open(filename, "wb")
-	if file == nil then return false end
+function write_binary_unsafe(filename, new_content)
+	local file, err = io.open(filename, "wb")
+	if file == nil then return false, err end
 	file:write(new_content)
 	file:close()
 	return true
@@ -61,24 +61,23 @@ function ensure_content(filename, ensured_content)
 end
 
 function append(filename, new_content)
-	local file = io.open(filename, "a")
-	if file == nil then return false end
+	local file, err = io.open(filename, "a")
+	if file == nil then return false, err end
 	file:write(new_content)
 	file:close()
 	return true
 end
 
 function exists(filename)
-	local file = io.open(filename, "r")
-	if file == nil then return false end
+	local file, err = io.open(filename, "r")
+	if file == nil then return false, err end
 	file:close()
 	return true
 end
 
 function create_if_not_exists(filename, content)
 	if not exists(filename) then
-		write(filename, content or "")
-		return true
+		return write(filename, content or "")
 	end
 	return false
 end