Give a better name to STOP_REASON_ERROR

It indicates crosvm failed to start the VM, so
STOP_REASON_START_FAILED.

Slightly gratuitously I also renamed the internal DeathReason - mainly
because this is just a much clearer name.

I didn't change the statslog version, because that would just be too
complicated.

Bug: 261037705
Test: atest MicrodroidTests
Change-Id: I5effdba86ea49ff919d82810926fb14ccbbd8499
diff --git a/virtualizationservice/src/atom.rs b/virtualizationservice/src/atom.rs
index 9c74d1e..85e37af 100644
--- a/virtualizationservice/src/atom.rs
+++ b/virtualizationservice/src/atom.rs
@@ -182,7 +182,7 @@
                 DeathReason::KILLED => vm_exited::DeathReason::Killed,
                 DeathReason::UNKNOWN => vm_exited::DeathReason::Unknown,
                 DeathReason::SHUTDOWN => vm_exited::DeathReason::Shutdown,
-                DeathReason::ERROR => vm_exited::DeathReason::Error,
+                DeathReason::START_FAILED => vm_exited::DeathReason::Error,
                 DeathReason::REBOOT => vm_exited::DeathReason::Reboot,
                 DeathReason::CRASH => vm_exited::DeathReason::Crash,
                 DeathReason::PVM_FIRMWARE_PUBLIC_KEY_MISMATCH => {