Upgrade AskSage error reporting and portability

In particular pass location of curl (used in sage call) from site.conf since the location can be different on different machines.
Create error messages when debug is set to 1
Merge pull request #437 from mgage/develop_fixsage

Upgrade AskSage error reporting and portability
Merge branch 'develop' of https://github.com/openwebwork/webwork2 into ww3