Plugin infrastructure swallows underlying error message

[bbockelm]

MIT recently reported an issue with a plugin that resulted in this following error message:

190627 14:12:04 25911 secgsi_LoadAuthzFun: Unable to load authzlib plugin /usr/lib64/libXrdLcmaps.so
190627 14:12:04 25911 secgsi_LoadAuthzFun: could not find 'XrdSecgsiAuthzFun()' in /usr/lib64/libXrdLcmaps.so

From here: https://github.com/xrootd/xrootd/blob/master/src/XrdSecgsi/XrdSecProtocolgsi.cc#L5431

Unfortunately, this error message is completely misleading - XrdSecgsiAuthzFun certainly exists in the library. However, there's a loader error that I suspect was noted in the dlerror message.

Could we ensure that, on failure, the contents of dlerror are logged? Would have helped immensely in debugging this issue.