Error message not shown when using an invalid xlsx file, and unable to do further commands

[VaishakAnand]

Issue: Serenity does not give any form of error / warning when an invalid xlsx file (a file that does not follow the LumiNUS downloaded file formatting). We are then unable to do the add new lesson command.

Steps to reproduce:

1. Remove the student info from the sample file and change some fields as such:

2. Add a new tutorial group using that file, using addgrp grp/G09 path/CS2101_G09.xlsx

3. Attempt to add a new lesson in that Tutorial group, eg addlsn grp/G09 lsn/4-2. (fails, and no error message is shown. No message at all)

4. try to do an invalid command, eg addgrp.

5. do step 3 again, and the error message from the previous step remains, which might be confusing.

Suggestions:

• Perhaps the excel file could be validated first, if possible.
• If no change is being made, then perhaps another error message could be shown (like in this case, me, the user thought that my command was incorrect and so kept retrying the command).