aeson 0.10's better error messages include useless information

[bergmark]

The json-schema test-suite is broken with aeson-0.10 (dunno why travis built with an older aeson and we didn't see this). The breakage is luckily just because error messages changed.

      expected: Left "when expecting a Int, encountered Boolean instead"
       but got: Left "Error in $: expected Int, encountered Boolean"

I'll fix the test-suite now, but we may want to think about improving the message since api users may see this error and $ is nonsense to them.