Fix exception handling

[Daft-Freak]

This fixes a issue that results in the backlight getting disabled with some firmware builds and a lua script containing an error.

More explanation to make that sound less absurd:

• lua_pcall /lua_error use setjmp/longjmp
• newlib's setjmp/longjmp don't save/restore floating point regs
• sleep_fade in the firmware is a float
• incorrect float registers break sleep updates (ends up removing the clamp to 1.0)
• out of bounds sleep_fade results in invalid PWM values for backlight
• /me gets very confused and spends ages debugging

So here's some custom exception handling that saves more registers and seems to work...