Add error handling for flash code

[PatrickKa]

Description

Use the Experimental Outcome library for proper error handling in the flash code.

[PatrickKa]

Think about whether this actually makes sense. If communication with the flash can fail, but we have no way to recover, error handling doesn't really make sense. Well, maybe we can increment an error counter or something.

[PatrickKa]

Blocked by #237

[PatrickKa]

No longer blocked by #237 because error handling for SPI communication works on a higher level now with a supervisor thread. This means that the flash code can assume that the SPI functions always work. It only needs to take care of other possible issues that can arise. I am not sure if they exist, though. One needs to go through the code and think about it.