Human readable OSError messages for low flash devices

[godlygeek]

#837 implemented human readable OSError messages, but excluded low-flash devices (ones with BOARD_FLASH_SIZE <= 192000).

This PR improves the situation on those devices:

Adafruit CircuitPython 3.0.0-beta.0-32-gd6ff451-dirty on 2018-06-01; Adafruit Gemma M0 with samd21e18
>>> for i in range(32): print(OSError(i))
...
[Errno 0]
[Errno 1] EPERM
[Errno 2] ENOENT
[Errno 3]
[Errno 4]
[Errno 5] EIO
[Errno 6]
[Errno 7]
[Errno 8]
[Errno 9]
[Errno 10]
[Errno 11] EAGAIN
[Errno 12] ENOMEM
[Errno 13] EACCES
[Errno 14]
[Errno 15]
[Errno 16]
[Errno 17] EEXIST
[Errno 18]
[Errno 19] ENODEV
[Errno 20]
[Errno 21] EISDIR
[Errno 22] EINVAL
[Errno 23]
[Errno 24]
[Errno 25]
[Errno 26]
[Errno 27]
[Errno 28]
[Errno 29]
[Errno 30]
[Errno 31]

On devices with low flash, we still won't create the uerrno module. Instead, we'll create a utility method that knows how to map each of the errno's that we can raise to its name. In order to save flash, we also prune the list of errno's that the utility method must know about down to just the subset that can be raised on atmel-samd.

[tannewt]

Yay! This looks great. Nice and clean. Thank you!