makeparallel: print path on exec failure

If ninja is missing makeparallel prints an unhelpful error:
out/host/linux-x86/bin/makeparallel: exec failed: No such file or directory
which suggests that makeparallel is what is missing, not what is
reporting the error.  Print the path passed to exec as well.

Change-Id: Ic04c1cde6da9c3a974b5c43e3fb3bdb6da6605f7

tools/makeparallel/makeparallel.cpp

@@ -343,7 +343,7 @@ int main(int argc, char* argv[]) {
     // child
     int ret = execvp(path, args.data());
     if (ret < 0) {
-      error(errno, errno, "exec failed");
+      error(errno, errno, "exec %s failed", path);
     }
     abort();
   }