Improves EC error handling

Adds option for detailed or generic error message
display in the settings panel, and uses this
option to display error messages accordingly.

Also fixes error handling problems left over
from original Woo extension within the DECP
request.  This now redirects back to the cart
page to display the error instead of the order page
which looks like the order was completed.

Resolves #16
Merge branch 'ec_error_handling' into dev