Give more errors for specific failure conditions

Some failure conditions implicitly fail by failing some other check.
But the error messages are more helpful if they say explicitly what
actually caused the failure, so add those as failure conditions and
errors.

src/rpc/util.cpp

@@ -720,7 +720,7 @@ std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, Fl
     std::string error;
     auto desc = Parse(desc_str, provider, error);
     if (!desc) {
-        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Invalid descriptor '%s', %s", desc_str, error));
+        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, error);
     }
     if (!desc->IsRange()) {
         range.first = 0;