<artwork> src and content confusion

[miekg]

Section 2.5

Alternatively, the "src" attribute allows referencing an external
graphics file, such as a vector drawing in SVG or a bitmap graphic
file, using a URI. In this case, the textual content acts as a
fallback for output representations that do not support graphics;
thus, it ought to contain either (1) a "line art" variant of the
graphics or (2) prose that describes the included image in sufficient
detail.

And later:

It is an error to have both a "src" attribute and content in the
<artwork> element.

[paulehoffman]

Fixed by #43