Revert "Add support for Coveralls"

This reverts commit 46392f288943cd348d9ba7cc55be13437c295358.

This is being reverted until the question at
<http://stackoverflow.com/questions/21549912/how-can-i-post-code-coverage-from-a-combined-c-javascript-node-js-addon-to-cov>
is resolved.
Fix incorrect routine string in error message
If no error message is specified use the error category instead

This mimics the behaviour of the core mapserver code.  For example,
this makes it possible to match error output from the issue at
geo-data/node-mapserv#13 with that produced when replicating the
behaviour using `shp2img`.