Improve error handling in completeCalendar to avoid panic

[mickmister]

PR #7 was created to be clear about configuring the plugin properly, as a remedy to avoid a panic in the plugin. The plugin should instead be resilient to this and handle the case of this error. See this comment for details #7 (comment)

There was an index out of range error occurring here:

mattermost-plugin-google-calendar/server/api.go

Line 83 in f85d05b

userId := strings.Split(state, "_")[1]

[mickmister]

Ideally the root cause of the problem is solved, instead of just avoiding the panic. The configuration issue should be brought to the attention of the user when they initially try to connect.