init: Check DecodeUid() result and use error string

Check the result of DecodeUid() and return failure when uids/gids are
unable to be decoded.

Also, use an error string instead of logging directly such that more
context can be added when decoding fails.

Bug: 38038887
Test: Boot bullhead
Test: Init unit tests
Change-Id: I84c11aa5a8041bf5d2f754ee9af748344b789b37
diff --git a/init/util_test.cpp b/init/util_test.cpp
index ac23a32..a24185b 100644
--- a/init/util_test.cpp
+++ b/init/util_test.cpp
@@ -115,10 +115,21 @@
     EXPECT_STREQ("2ll2", s2.c_str());
 }
 
-TEST(util, decode_uid) {
-  EXPECT_EQ(0U, decode_uid("root"));
-  EXPECT_EQ(UINT_MAX, decode_uid("toot"));
-  EXPECT_EQ(123U, decode_uid("123"));
+TEST(util, DecodeUid) {
+    uid_t decoded_uid;
+    std::string err;
+
+    EXPECT_TRUE(DecodeUid("root", &decoded_uid, &err));
+    EXPECT_EQ("", err);
+    EXPECT_EQ(0U, decoded_uid);
+
+    EXPECT_FALSE(DecodeUid("toot", &decoded_uid, &err));
+    EXPECT_EQ("getpwnam failed: No such file or directory", err);
+    EXPECT_EQ(UINT_MAX, decoded_uid);
+
+    EXPECT_TRUE(DecodeUid("123", &decoded_uid, &err));
+    EXPECT_EQ("", err);
+    EXPECT_EQ(123U, decoded_uid);
 }
 
 TEST(util, is_dir) {