Tidy eth1/deposit contract logging (#3397)

## Issue Addressed

Fixes an issue identified by @remyroy whereby we were logging a recommendation to use `--eth1-endpoints` on merge-ready setups (when the execution layer was out of sync).

## Proposed Changes

I took the opportunity to clean up the other eth1-related logs, replacing "eth1" by "deposit contract" or "execution" as appropriate.

I've downgraded the severity of the `CRIT` log to `ERRO` and removed most of the recommendation text. The reason being that users lacking an execution endpoint will be informed by the new `WARN Not merge ready` log pre-Bellatrix, or the regular errors from block verification post-Bellatrix.

common/fallback/src/lib.rs

@@ -45,7 +45,7 @@ impl<T> Fallback<T> {
     {
         match error {
             FallbackError::AllErrored(v) => format!(
-                "All fallback errored: {}",
+                "All fallbacks errored: {}",
                 join(
                     zip(self.servers.iter().map(f), v.iter())
                         .map(|(server, error)| format!("{} => {:?}", server, error)),