Test for upload issues

[perrygeo]

Ultimately we want something hooked into the upload process that will check for red flags and report meaningful errors to the user.

As a first step, we could add a command to this repo which checked for:

• projection
• tiling
• format
• compression
• nodata/alpha issues
• ???

[sgillies]

@perrygeo colormaps and 16 or 32-bit data types have been stumbling blocks, too.

The key to the recent problem would be to determine if a nodata value is shadowing an alpha channel. Quick check for that would be something like ColorInterp.alpha in [src.colorinterp(i) for i in src.indexes] and MaskFlags.nodata in itertools.chain(*self.mask_flag_enums).