Let dispatcher raise error when a DTM broadcast fails

There already is a PG_TRY() ... PG_CATCH() block to handle such errors
in the DTM retry logic.  This change makes it easier to test the error
handling in the retry logic.

The patch also fixes a bug in dispatcher that invoked CopyErrorData
interface incorrectly, without switching to a memory context other that
ErrorContext.

src/backend/cdb/cdbtm.c

@@ -791,7 +791,7 @@ doNotifyingCommitPrepared(void)
 	{
 		succeeded = doDispatchDtxProtocolCommand(DTX_PROTOCOL_COMMAND_COMMIT_PREPARED, /* flags */ 0,
 												 currentGxact->gid, currentGxact->gxid,
-												 &badGangs, /* raiseError */ false,
+												 &badGangs, /* raiseError */ true,
 												 &direct, NULL, 0);
 	}
 	PG_CATCH();
@@ -839,7 +839,7 @@ doNotifyingCommitPrepared(void)
 			succeeded = doDispatchDtxProtocolCommand(
 													 DTX_PROTOCOL_COMMAND_RETRY_COMMIT_PREPARED, /* flags */ 0,
 													 currentGxact->gid, currentGxact->gxid,
-													 &badGangs, /* raiseError */ false,
+													 &badGangs, /* raiseError */ true,
 													 &direct, NULL, 0);
 		}
 		PG_CATCH();

src/backend/cdb/dispatcher/cdbdispatchresult.c

@@ -557,6 +557,15 @@ cdbdisp_dumpDispatchResult(CdbDispatchResult *dispatchResult)
 	return NULL;
 }
 
+/*
+ * The returned error object is allocated in TopTransactionContext.
+ * 
+ * Caution: do not use the returned object across transaction boundary.
+ * Current usages of this API are such that the returned object is either
+ * logged using elog() or rethrown, both within a transaction context, at the
+ * time of finishing a dispatched command.  The caution applies to future uses
+ * of this function.
+ */
 ErrorData *
 cdbdisp_get_PQerror(PGresult *pgresult)
 {
@@ -633,7 +642,11 @@ cdbdisp_get_PQerror(PGresult *pgresult)
 	if (fld)
 		errcontext("%s", fld);
 
-	return errfinish_and_return(0);
+	Assert(TopTransactionContext);
+	MemoryContext oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+	ErrorData *edata = errfinish_and_return(0);
+	MemoryContextSwitchTo(oldcontext);
+	return edata;
 }
 
 /*