Change punctation and capitalization in warnings

...and make `alternativeactionstr` an optional argument. Partly
addresses #998.

Example:

    >>> ICRSCoordinates('00:60 00:60', unit=('deg', 'deg'))
    WARNING: IllegalMinuteWarning: 'minute' was found  to be '60', which is
    not in range [0,60). Treating as 0 min, +1 hr/deg
    [astropy.coordinates.angle_utilities]
    <ICRSCoordinates RA=1.00000 deg, Dec=1.00000 deg>