Move startRBE error handling to the main goroutine

When called from the startRBE goroutine, the panic ctx.Fatalf triggered
by is unhandled and causes the process to exit without cleaning up the
terminal.  Move most of the quick checks to a checkRBERequirements
function called on the main goroutine so they can be reported
immediately before running Soong and Kati, and recover any panics
from startRBE and report them after waiting on rbeCh.

Bug: 293501187
Test: gcertdestroy && m nothing
Change-Id: I62c84c93ab0a7f0e4f2ab2cc64b22e2070dd6e4c