Revert downgrading of log error->warn->error

Add more guidance to CheckSingleLedgerHeaderWork failure case to assure operators this is not a fatal error, advises them to notify operator of faulty archives is issue persist

src/historywork/CheckSingleLedgerHeaderWork.cpp

@@ -84,9 +84,16 @@ CheckSingleLedgerHeaderWork::doWork()
     }
     else if (mGetLedgerFileWork->getState() != State::WORK_SUCCESS)
     {
-        CLOG_WARNING(History,
-                     "Failed to download ledger checkpoint {} from archive {}",
-                     mFt->baseName_gz(), mArchive->getName());
+        CLOG_ERROR(
+            History,
+            "Failed to download ledger checkpoint {} from archive {}. This may "
+            "be due to transient network issues, or the archive may be missing "
+            "the checkpoint. stellar-core will try to download the file from "
+            "another archive. This error *does not* necessarily require a "
+            "configuration change to remove the archive. If you repeatedly see "
+            "this error, please notify the operator of the archive that you "
+            "are observing gaps.",
+            mFt->baseName_gz(), mArchive->getName());
         return mGetLedgerFileWork->getState();
     }