more detailed logging when RasterSource falls through

[vpipkt]

The matcher here displays a warning about specific URI unable to be read, but debugging would benefit from a more detailed logging of the various conditions involved. Meaning: what are the conditions actually encountered.

Also... the wording of the exception is maybe too strong: "Reading of X is not supported". It may be more appropriate to couch that in terms of availability of GDAL and if so the formats GDAL has access to. A clarifying point: reading of song.mp3 is not supported; but if gdal is not configured correctly there is an error with raster.mrf but it could be configured and read.