Handle error cases in the TTVFast code by raising python exceptions

[simonrw]

For example:

if(a<0.0) {

    printf("hyperbolic orbit %lf\n", a);

    exit(-1);

  }

[simonrw]

The trouble is that this code is in the TTVFast module itself rather than the Python wrapper. This would mean either:

• trying to notice when the library code has exited somehow
• patching the external code
  • during the compilation phase
  • in the upstream repository
  • do we fork? or
  • do we contribute a patch upstream?

[simonrw]

See the issue in TTVFast-python-compat: https://github.com/mindriot101/TTVFast-python-compat/issues/1

[simonrw]

The issue has now been merged to upstream, so we can track master again!