separate non-zero exit code for doppelganger detection (#3728)

beacon_chain/gossip_processing/eth2_processor.nim

@@ -281,7 +281,11 @@ proc checkForPotentialDoppelganger(
           validatorIndex,
           validatorPubkey,
           attestation = shortLog(attestation)
-        quit QuitFailure
+
+        # Avoid colliding with
+        # https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Process%20Exit%20Codes
+        const QuitDoppelganger = 1031
+        quit QuitDoppelganger
 
 proc attestationValidator*(
     self: ref Eth2Processor, src: MsgSource,

docs/the_nimbus_book/src/options.md

@@ -9,7 +9,7 @@ You can pass any `nimbus_beacon_node` options to the `prater` and `mainnet` scri
 To see a list of the command line options availabe to you, with descriptions, run:
 
 ```
-build/./nimbus_beacon_node --help
+build/nimbus_beacon_node --help
 ```
 
 You should see the following output:
@@ -128,3 +128,10 @@ num-threads = 0
 trusted-node-url = "http://192.168.1.20:5052"
 ```
 
+# Exit Codes
+
+| Exit code | Description |
+|-----------|---------|
+| 0 |  Successful exit |
+| 1 | Generic failure or unspecified error |
+| 1031 | Doppelganger detection; one might prefer not to restart automatically |