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 158c9b6ba18b39f424bd524fceb66f3fec0d1616.