[logging/documentation] Remove reference to AcceptToMemoryPool from error string

User-facing error messages should not leak internal implementation
details like function names. Update the MEMPOOL_REJECTED error string
from "Transaction rejected by AcceptToMemoryPool" to the more generic
"Transaction rejected by mempool". Also update the MEMPOOL_ERROR error
message from "AcceptToMemoryPool failed" to the more precise "Mempool
internal error" since this error indicates and internal (e.g.
logic/hardware/etc) failure, and not a transaction rejection.

Author: jnewbery

src/util/error.cpp

@@ -20,9 +20,9 @@ bilingual_str TransactionErrorString(const TransactionError err)
         case TransactionError::P2P_DISABLED:
             return Untranslated("Peer-to-peer functionality missing or disabled");
         case TransactionError::MEMPOOL_REJECTED:
-            return Untranslated("Transaction rejected by AcceptToMemoryPool");
+            return Untranslated("Transaction rejected by mempool");
         case TransactionError::MEMPOOL_ERROR:
-            return Untranslated("AcceptToMemoryPool failed");
+            return Untranslated("Mempool internal error");
         case TransactionError::INVALID_PSBT:
             return Untranslated("PSBT is not well-formed");
         case TransactionError::PSBT_MISMATCH: