Move buffer creation in s:ExtraditeLoadCommitData

Was performed before obtaining list of log entries for the current
file.  Not this is done after.
Print error message on empty list of log entries

Otherwise one gets a bunch of errors from Vim related to accessing
inexistent entries of `b:extradata_list` list.
Merge pull request #20 from xaizek/master

Print error message on empty list of log entries