Unhelpful error messages

[mikeboers]

From @parke:

I switched from ctr mode to cbc mode, and got this error:

tomcrypt.LibError: Invalid argument provided.

I guess CBC mode needs the input to be a multiple of the block size. A more descriptive error message would have been helpful.

[parke]

A quicker (interim?) fix would be to document which ciphers require block length inputs.

[mikeboers]

That is something that can be easily done.

In general, I would like to rework the error handling. Specifically, I think I would like to build a new exception tree, instead of just having Error and LibError.

At the most extreme, we would have an exception for each of the 2 dozen(ish) errors that the libtomcrypt gives us, and they would also inherit from the standard exceptions (usually ValueError or RuntimeError) as appropriate.

[mikeboers]

For reference, the modes which require a block sized input are: ecb, cbc, and lrw.