feat: print an error message when ~/.gcalcli_oauth deserialization fails

[adrien-n]

At the moment, only a backtrace is printed and it doesn't make what's going on obvious.

This is especially important now as the migration from oauth2client to google-auth comes with a format change for this file and anyone who had been using gcalcli successfully before is likely to encounter this error.

I've tested this change the following way in a container:

# echo x > .gcalcli_oauth
# gcalcli list
Couldn't parse /root/.gcalcli_oauth.
The file may be corrupt or be incompatible with this version of gcalcli. It probably has to be removed and provisioning done again.
[backtrace]

This doesn't cover the file being empty but a) this is quite unlikely to happen, b) the cause would be completely different; moreover the exception is quite clear: EOFError: Ran out of input

Helps with #691.

[dbarnett]

Love it, thanks!

Merging...