[lldb] Use SWIG_fail in python-typemaps.swig (NFC)

When attempting to use SWIG's `-builtin` flag, there were a few compile
failures caused by a mismatch between return type and return value. In those
cases, the return type was `int` but many of the type maps assume returning
`NULL`/`nullptr` (only the latter caused compile failures).

This fix abstracts failure paths to use the `SWIG_fail` macro, which performs
`goto fail;`. Each of the generated functions contain a `fail` label, which
performs any resource cleanup and returns the appropriate failure value.

This change isn't strictly necessary at this point, but seems like the right
thing to do, and for anyone who tries `-builtin` later, it resolves those
issues.

Differential Revision: https://reviews.llvm.org/D133961