Give a better error message if libwebp is not installed on Linux

[robpvn]

Right now it's the standard uncaught error dialogue with a DLLException warning.

[robpvn]

On a related note, we should see if the DLLImport can be more robust. Case in point: On OpenSUSE libraries are named libwebp.so.0 for some silly reason, so I had to create a symlink (in usr/lib64, beside libwebp.so.0) named libwebp.so for Mono's loader to be able to find it. There might be ways of specifying alternate names in the import, we should look at that.

[cameronwhite]

Does f731412 work for you? The mono docs seemed to suggest that "webp" was a better choice than "libwebp".

[cameronwhite]

There's also now a proper error dialog in 9d24246.

It's good that we're developing some sample addins ahead of time, since I've already found a few API changes/additions that needed to be made.

[robpvn]

Well, the error message works alright anyway... :P
I'm not on my OpenSUSE laptop, so I can't check there, but on my Ubuntu 12.10 desktop machine it's not finding the library.

[cameronwhite]

It looks like Mono has a mechanism for specifying alternate names (http://pkg-mono.alioth.debian.org/cli-policy/ch-mono.html).

On my system, I have the symlinks libwebp.so and libwebp.so.4 pointing to libwebp.so.4.0.2