Fix searching issues with classifier-reborn

[tra38]

This pull request should handle issues #64 and #75, mostly by throwing up human-readable error messages. Error messages aren't cool, but they're more graceful than programs crashing.

Issue #64 is caused by adding "invalid" documents beforehand, and we resolve this by rejecting those "invalid" documents before they could cause problems later on for the end-user. If you try to search a corpus of documents that include "invalid" documents, the program will crash. If you try to find related phrases, it won't crash and you'll still get results. However, I would still recommend rejecting "invalid" documents outright because these invalid documents adds noise that can narrowly impact the LSI's results.

Issue #75 is caused by attempting to search through a corpus for a term that does not exist within the corpus. We resolve this issue by rejecting the search term outright. The user here knows that no document has the search term in question, and will be able to move on with their lives. This bug fix only works if you have GSL...otherwise, you just get the built-in "Cannot Normalize Zero Vector" error (which isn't a user-friendly error message, but it does stop invalid searches).

Let me know if these changes are appropriate or if you have your own suggestions.

[Ch4s3]

Just saw this pull request. Tanks a ton!