added the actual number of sheets in the out of bounds exception
added the requested index to the out of bounds exception
Still improving the out of bounds error messages in the exceptions.
minor architecture improvement
  - used strtoupper to get rid of case sensitive name checks
  - added "else if", it's not nessecary to check for the second "if", if
the first one was allready true
Improved the phrasing of the error messages.
replaced tabs by 4 spaces
replaced tabs by 4 spaces
reformated the docblock comments (looked a litle strange after
replacing the tabs)
replaced tabs by 4 spaces
Merge branch 'develop' into experimental
Merge remote-tracking branch 'origin/experimental' into develop