Improve error message and hint for ALTER COLUMN TYPE can't-cast failure.

We already tried to improve this once, but the "improved" text was rather
off-target if you had provided a USING clause.  Also, it seems helpful
to provide the exact text of a suggested USING clause, so users can just
copy-and-paste it when needed.  Per complaint from Keith Rarick and a
suggestion from Merlin Moncure.

Back-patch to 9.2 where the current wording was adopted.

src/backend/commands/tablecmds.c

@@ -7266,11 +7266,26 @@ ATPrepAlterColumnType(List **wqueue,
 										  COERCE_IMPLICIT_CAST,
 										  -1);
 		if (transform == NULL)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATATYPE_MISMATCH),
-			  errmsg("column \"%s\" cannot be cast automatically to type %s",
-					 colName, format_type_be(targettype)),
-					 errhint("Specify a USING expression to perform the conversion.")));
+		{
+			/* error text depends on whether USING was specified or not */
+			if (def->raw_default != NULL)
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("result of USING clause for column \"%s\""
+								" cannot be cast automatically to type %s",
+								colName, format_type_be(targettype)),
+						 errhint("You might need to add an explicit cast.")));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("column \"%s\" cannot be cast automatically to type %s",
+								colName, format_type_be(targettype)),
+				/* translator: USING is SQL, don't translate it */
+					   errhint("You might need to specify \"USING %s::%s\".",
+							   quote_identifier(colName),
+							   format_type_with_typemod(targettype,
+														targettypmod))));
+		}
 
 		/* Fix collations after all else */
 		assign_expr_collations(pstate, transform);