Placeholder resolutions

[fcollman]

When users try to go from neuroglancer>cloudvolume with flywire they are tempted to use naive code like this

im_cv= cloudvolume.CloudVolume('precomputed://https://bossdb-open-data.s3.amazonaws.com/flywire/fafbv14')

but this will give them a 4,4,40 mip which is blank, and no warnings as to what is going on.

the info file has a key= 'placeholder', but cloudvolume isn't trying to detect this to auto-avoid configuring its mip to that, or at a minimum warning the user that this might be a placeholder mip.