Move custom Mathematical errors inline to C

Three specific errors for the C parsing, rather than relying on the Ruby standard ones:

``` c
// Raised when the contents could not be parsed
static VALUE rb_eParseError;
// Raised when the SVG document could not be created
static VALUE rb_eDocumentCreationError;
// Raised when the SVG document could not be read
static VALUE rb_eDocumentReadError;
```

@bkeepers I think you also suggested that begin/rescue block here be removed: https://github.com/gjtorikian/mathematical/blob/f5de7533c7253262a4de74de165622994fe3743b/lib/mathematical/render.rb#L24-L31

What's the rationale for that? That handlers of the library should best rescue the errors?