feature request: Add a clearer error message, if the datatype for the sheetname is incorrect.

[fwitte]

Feature Request

From time to time I teach python beginners on how to write data from pandas to excel. Quite often they happen to provide a sheetname that is of type int. The errormessage shown, i.e. that a int type variable has no len might be clear for experienced users, but might be difficult to understand for beginners. Therefore, I opened a mini PR, introducing a type check and a clear error message for wrong type sheetnames.

[jmcnamara]

Thanks for the suggestion.

However I don't think this is a common enough error to merit its own exception so I will pass on the PR.