db-synthesizer: explain what "does not look like a ChainDB" means (#1206)

Motivation is to make the failure less confusing. We might also want to
relax the `checkIsDB` predicate to be more lenient/less overeager in the
future.

Author: amesgen

ouroboros-consensus-cardano/src/unstable-cardano-tools/Cardano/Tools/DBSynthesizer/Run.hs

@@ -183,4 +183,6 @@ preOpenChainDB mode db =
             OpenCreateForce | isChainDB ->
                 removePathForcibly db >> create
             _ ->
-                fail $ loc ++ " is non-empty and does not look like a ChainDB. Aborting."
+                fail $ loc ++ " is non-empty and does not look like a ChainDB"
+                    <> " (i.e. its entries are not exactly 'immutable'/'ledger'/'volatile')."
+                    <> " Aborting."