os: make read_lines() return ?[]string

vlib/os/os.v

@@ -196,8 +196,7 @@ fn vfopen(path, mode string) *C.FILE {
 }	
 
 // read_lines reads the file in `path` into an array of lines.
-// TODO return `?[]string` TODO implement `?[]` support
-pub fn read_lines(path string) []string {
+pub fn read_lines(path string) ?[]string {
 	mut res := []string
 	mut buf_len := 1024
 	mut buf := malloc(buf_len)
@@ -205,9 +204,7 @@ pub fn read_lines(path string) []string {
 	mode := 'rb'
 	mut fp := vfopen(path, mode)
 	if isnil(fp) {
-		// TODO
-		// return error('failed to open file "$path"')
-		return res
+		return error('read_lines() failed to open file "$path"')
 	}
 
 	mut buf_index := 0
@@ -217,7 +214,7 @@ pub fn read_lines(path string) []string {
 			buf_len *= 2
 			buf = C.realloc(buf, buf_len)
 			if isnil(buf) {
-				panic('Could not reallocate the read buffer')
+				return error('could not reallocate the read buffer')
 			}
 			buf_index = len
 			continue
@@ -235,8 +232,10 @@ pub fn read_lines(path string) []string {
 	return res
 }
 
-fn read_ulines(path string) []ustring {
-	lines := read_lines(path)
+fn read_ulines(path string) ?[]ustring {
+	lines := read_lines(path) or {
+		return err
+	}	
 	// mut ulines := new_array(0, lines.len, sizeof(ustring))
 	mut ulines := []ustring
 	for myline in lines {