minor fix to exception throwing

[basil-inc]

Got the error "Class 'mikehaertl\pdftk\Exception' not found" upon throwing the exception

[basil-inc]

@mikehaertl by the way: why do you limit the page handles to the letters A-Z?
I am trying to use your library to concatenate a bunch of graphs into a larger PDF, but their number will probably exceed the letters in the alphabet. Why can't it accept any string?

[basil-inc]

PS: I really like your code. I also saw you're a contributor to the Yii2 framework, which I am using in my current project. Thanks for sharing your work!

[mikehaertl]

why do you limit the page handles to the letters A-Z?

AFAIK that's a limitation of pdftk versions 1.x. I've tried to use more letters and it didn't work.

I also want to add support for 2.x versions, but not quite sure yet, what's the best way here. Need to check all the differences first.