Make errors from regfile_iter_new() non-fatal.

The callers of the regfile iter API already handle the case of no
epigrams or tag expressions just fine. In particular, the stats
command can still print meaningful output if the tag expression
directory does not exist, which is a perfectly valid setup.

This patch modifies regfile_iter_new() to no longer abort but merely
print a log message and return NULL if opendir() fails. The various
accessors of the regfile iter API have to be adjusted to deal with
NULL pointers but no changes to tfortune.c are required.