Only use the message part of the warning/errors

[antgonza]

Currently, after the Python3 update, the warning messages are reported like this:

{message : QiitaDBWarning('There are no differences between the data stored in the DB and the new data provided',), category : 'QiitaDBWarning', filename : '/home/qiita/qiita/qiita_db/metadata_template/base_metadata_template.py', lineno : 1351, line : None}
{message : ParserWarning('Both a converter and dtype were specified for column sample_name - only the converter will be used',), category : 'ParserWarning', filename : '/home/qiita/qiita/qiita_db/metadata_template/util.py', lineno : 158, line : None}
{message : QiitaDBWarning('The following columns have been added to the existing template: runid',), category : 'QiitaDBWarning', filename : '/home/qiita/qiita/qiita_db/metadata_template/base_metadata_template.py', lineno : 856, line : None}

should only use the message part ...