Factor common svn_log_message_receiver_t code into pyify_log_message()
Factor out ra_get_log_prepare() to share with get_log() and iter_log()
Factor common error cleanup code with goto

Should fix numerous leaks of APR pools, Python object references, and the RA
busy flag.
Add missing exception handling for PyString_FromString() calls

In Python 3 these will all involve UTF-8 decoding, which can easily trigger
exceptions for invalid UTF-8 data. For instance this happened when running
the tests and a commit was made with the “svn:log” string accidentally not
terminated. The string had random non UTF-8 data appended to it.