Revert "Generate error constants as XCB_BAD_*, similar to Xlib."
Since several extensions named their errors like "BadFoo", this patch results in names like XCB_EXT_BAD_BAD_FOO, which is really awful. Those extensions are already kind of awful, as they produce structure names like xcb_ext_bad_foo_error_t, which is redundant. A patch that removes "Bad" from the XML extension descriptions, while maintaining API and ABI compatibility in XCB, is needed before this patch can be released. This reverts commit 158c9b6b.