Handle invalid svgs gracefully

[dlew]

Addresses #7

[hamidp]

I might be missing something but to me it seems like an invalid svg is like invalid code, so the right behavior would be to error out.

[dlew]

I'm taking a note from the standard Android build process which does keep processing files even if they're invalid (in some cases). Not sure what the best practice here is though.

At the very least, this gives us points at which we could throw exceptions instead of logging warnings.

[hamidp]

Ah, :+1: