exception handling for hexpansion I2C

[Robotmad]

Fix for Issue #188.

If hexpansion is unplugged during an I2C operation an OSError exception is generated which was not being trapped leading to a crash. i2c read and write operations now protected and abort the transfer, not attempting any further bytes.

[kliment]

Far as I can see, this can result in incomplete reads that then get interpreted as a filesystem. We should probably handle this error one level higher

[ChrisDick]

I've given this a try and it does lead to incomplete file reads that are then run.

Traceback (most recent call last):
File "main.py", line 43, in
File "system/scheduler/init.py", line 243, in run_forever
File "asyncio/core.py", line 1, in run_until_complete
File "asyncio/core.py", line 1, in run_until_complete
File "asyncio/core.py", line 1, in run_until_complete
File "system/scheduler/init.py", line 239, in _main
File "asyncio/funcs.py", line 1, in gather
File "asyncio/core.py", line 1, in run_until_complete
File "system/eventbus.py", line 101, in run
File "asyncio/funcs.py", line 1, in gather
File "asyncio/core.py", line 1, in run_until_complete
File "system/hexpansion/app.py", line 254, in handle_hexpansion_insertion
File "system/hexpansion/app.py", line 216, in _mount_eeprom
File "system/hexpansion/app.py", line 142, in _launch_hexpansion_app
File "hexpansion_2/app.py", line 62
IndentationError: unindent doesn't match any outer indent level

there is no indentation problem with the file.