When PSM can't import a cert (root CA cert, subordinate CA cert, or web server cert), it does not display any error message. It just silently fails. This has been mentioned in the following bugs, whose purposes are on the underlying cause of the cert import failure. 1. Bug 221272: we can't import a .crt file with Macintosh line endings on Windows. 2. Bug 229565: we can't import certs with a lifetime over year 2049. (Reproducing this bug requires NSS 3.8.x or older, which does not support GeneralizedTime.)
The right place to possibly add error messages on failure is PSMContentDownloader.