init: do not log directly from read_file() and write_file()

Their callers may be able to add more context, so use an error string
to record the error.

Bug: 38038887
Test: boot bullhead
Test: Init unit tests
Change-Id: I46690d1c66e00a4b15cadc6fd0d6b50e990388c3
diff --git a/init/init_parser.cpp b/init/init_parser.cpp
index 620367a..1b31cf2 100644
--- a/init/init_parser.cpp
+++ b/init/init_parser.cpp
@@ -110,7 +110,9 @@
     LOG(INFO) << "Parsing file " << path << "...";
     Timer t;
     std::string data;
-    if (!read_file(path, &data)) {
+    std::string err;
+    if (!ReadFile(path, &data, &err)) {
+        LOG(ERROR) << err;
         return false;
     }