esp8266/modules: Fix fs_corrupted() to use start_sec not START_SEC.
esp8266/modules: Fix fs_corrupted() to use start_sec not START_SEC.

START_SEC was changed in e0905e85a7ad2961aa9192f6130565860e531ad3.

Also, update the error message to mention how to format the partition at
the REPL, and make the total message shorter to save a bit of flash.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython