workaround for the utf8 code points in the Sage banner
docbuilding: suppress warnings during latex build
Trac #15802: Sage banner and PDF docs

Add workarounds for the utf8 in the sage banner

URL: http://trac.sagemath.org/15802
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): John Palmieri
Trac #15800: PDF docs don't build

In #15727 we made sphinx errors hard errors. But building pdf
documentation requires less strict error checking, right now it fails
with
{{{
[cmd      ] /home/release/Sage/src/doc/en/reference/cmd/index.rst::
WARNING: unusable reference target found: ../genindex.html
Traceback (most recent call last):
  File "/home/release/Sage/src/doc/common/builder.py", line 83, in f
    execfile(sys.argv[0])
  File "/home/release/Sage/src/doc/common/custom-sphinx-build.py", line
185, in <module>
    sphinx.cmdline.main(sys.argv)
  File "/home/release/Sage/local/lib/python2.7/site-
packages/Sphinx-1.1.2-py2.7.egg/sphinx/cmdline.py", line 206, in main
    print >>error
  File "/home/release/Sage/src/doc/common/custom-sphinx-build.py", line
165, in write
    self._write(str)
  File "/home/release/Sage/src/doc/common/custom-sphinx-build.py", line
139, in _write
    self._log_line(line)
  File "/home/release/Sage/src/doc/common/custom-sphinx-build.py", line
108, in _log_line
    raise OSError(line)
OSError: [cmd      ]
/home/release/Sage/src/doc/en/reference/cmd/index.rst:: WARNING:
unusable reference target found: ../genindex.html
}}}

URL: http://trac.sagemath.org/15800
Reported by: vbraun
Ticket author(s): John Palmieri
Reviewer(s): Volker Braun