Don't write to EXRAM if it doesn't exist.

Previously, if a gamecube game wrote to an EXRAM address, dolphin would
segfault.
Panic on invalid reads/writes for non-mmu games.

Previously it would fall through to the mmu code path, and raise a dsi
exception, which it would never check for, so it would continue
executing code silently.
Merge pull request #991 from phire/dsi_should_crash

Panic if game does an invalid memory access.