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

ui/build/build.go

@@ -283,11 +283,16 @@ func Build(ctx Context, config Config) {
 	}
 
 	rbeCh := make(chan bool)
+	var rbePanic any
 	if config.StartRBE() {
 		cleanupRBELogsDir(ctx, config)
+		checkRBERequirements(ctx, config)
 		go func() {
+			defer func() {
+				rbePanic = recover()
+				close(rbeCh)
+			}()
 			startRBE(ctx, config)
-			close(rbeCh)
 		}()
 		defer DumpRBEMetrics(ctx, config, filepath.Join(config.LogsDir(), "rbe_metrics.pb"))
 	} else {
@@ -345,6 +350,11 @@ func Build(ctx Context, config Config) {
 	}
 
 	<-rbeCh
+	if rbePanic != nil {
+		// If there was a ctx.Fatal in startRBE, rethrow it.
+		panic(rbePanic)
+	}
+
 	if what&RunNinja != 0 {
 		if what&RunKati != 0 {
 			installCleanIfNecessary(ctx, config)