Add FAILED: prefix to RBE Fatal error message

This error message it currently being
logged to stdout which goes to build.log file . We want it to be logged to
build_error.log file instead so that downstream systems that need to run
based on the error message will continue to work.

Test: Tested by running `m toybox` and ensuring that FAILED: prefix is
at the beginning of the line.

Bug: b/177221705
Change-Id: Ie0c9acee2739acf149384e33804b5fca3273c24f