Andrew Ruthven bda3db6da8 Return a nicer error message if no user is found for Free/Busy via email
Previously a stack trace would be returned as the call to dav_name failed,
for example:

Exception [0] Can't calculate dav_name for unknown username
At line 382 of /usr/share/davical/inc/Principal.php
================= Stack Trace ===================
/usr/share/davical/htdocs/freebusy.php[49] Principal->dav_name()
2021-03-02 00:26:49 +13:00
..
2017-01-08 15:48:52 +01:00
2017-01-08 15:48:52 +01:00
2019-01-05 00:12:34 +13:00
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