Here's a test for exposing error messages; #279

This works as expected with the default error simplate, we just needed a
test for it. Next step is making sure the originally negotiated content
type gets passed through to the error simplate.
Teach error.spt about original media type

When you raise a Response you want to use any content type negotiation
that has already taken place when deciding which page of your error
simplates to use.
Merge branch 'master' into take-2-on-issue279
Clean up test suite around show_tracebacks

In local dev I had ASPEN_SHOW_TRACEBACKS configured, which caused my
local test suite run to differ from Travis, etc.
Convert error.spt to use stdlib_format exclusively

Instead of dropping back to stdlib_percent for the json section, keep
the sections consistent.
Merge pull request #288 from gittip/take-2-on-issue279

Take 2 on a fix for issue279