Get the exit policy from descriptors when available

Some complex exit policies are summarized as `reject 1-65535`. Since the consensus only contains the summary, calls to `can_exit_to()` incorrectly report `False`.

Examples from a current consensus:

```
03FF94D9E5001DD2290BC3B19FA7F59CE1E30279
10C0C888E4812BEFCD7B9449E5310A022FD1325B
296B2178FD742AB35AB20C9ADF04D5DFD3D407EB
65C35C03571307D7546D6978605A6B11B473F6EE
78C7C299DB4C4BD119A22B87B57D5AF5F3741A79
8887213ED36CE2810E4162E6254D3CF7324C3A1B
C4D41BFB33993F9D64699726A445283CE307E30C
CB2BEB364E07CF431819F6C5349555425C60C6F3
D897E7055BF702763BB88442F73D46CF7DBC0B6F
```