LP#1673870: ebook checkout fails on error message, not on empty due date

LP#1673870: ebook checkout fails on error message, not on empty due date

Previously, a successful ebook checkout would appear to fail if the
checkout response from the vendor API did not include a due date.  With
this commit, the checkout failure message is displayed only if Evergreen
returns an error message to the browser (i.e. the response object
contains an "error_msg" attribute).

Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>