build: fix hgrc manpage building with docutils 0.9
Since docutils 0.9, `roman` module has been moved from module directory root
(i.e. `site-packages/roman.py`) to `docutils.utils` module. Therefore `import
roman` statement should be wrapped in `try: ... except ImportError: ...` block
to handle importing correctly.
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!
[255]