Refine PNaCl pexe fetch UMA errors to know if the user aborted, etc.

ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc

Index: ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc
diff --git a/ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc b/ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc
index bd551ebcc7c7f1772430d398460a97ae8181f571..62ad2304d9e9388cae3a876d328b901170d1de11 100644
--- a/ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc
+++ b/ppapi/native_client/src/trusted/plugin/pnacl_translate_thread.cc
@@ -200,7 +200,10 @@ void PnaclTranslateThread::DoTranslate() {
     PLUGIN_PRINTF(("PnaclTranslateThread StreamEnd failed\n"));
     if (llc_subprocess_->srpc_client()->GetLastError() ==
         NACL_SRPC_RESULT_APP_ERROR) {
-      // The error string is only present if the error was sent back from llc
+      // The error string is only present if the error was sent back from llc.
+      // TODO(jvoung,dschuff): Be able to distinguish between things like:
+      //  - Out of memory.
+      //  - Bitcode doesn't match ABI.
       TranslateFailed(ERROR_PNACL_LLC_INTERNAL,
                       params.outs()[3]->arrays.str);
     } else {
@@ -271,6 +274,9 @@ bool PnaclTranslateThread::RunLdSubprocess(int is_shared_library,
                                        is_shared_library,
                                        soname.c_str(),
                                        lib_dependencies.c_str())) {
+    // TODO(jvoung,dschuff): Be able to distinguish between things like:
+    //  - Out of memory.
+    //  - Nexe has undefined symbols.
     TranslateFailed(ERROR_PNACL_LD_INTERNAL,
                     "link failed.");
     return false;