Improve failure message and description of dist target

This commit should clarify the fact that the ant dist target is expected to be
run from Bio-Formats as a submodule of openmicroscopy.git
Merge pull request #1476 from sbesson/ant_dist_improvements

Improve failure message and description of dist target
Update auto-generated documentation

Repository: openmicroscopy/bioformats
Already up-to-date.

Generated by BIOFORMATS-5.1-latest-docs-autogen#227 (http://ci.openmicroscopy.org/job/BIOFORMATS-5.1-latest-docs-autogen/227/)