more explicit NotImplementedError messages in read mode ?

[spolloni]

In wanting to set a GeoTIFF's CRS, I encountered this NotImplementedError when trying to run the following code:

with rasterio.open(filepath) as src:
    src.crs = "EPSG:3857"

Though in retrospect it is obvious the above will fail without explicitly specifying the proper mode , i.e. 'r+' in this case, I was momentarily thrown off by the error and assumed something was wrong with my approach. Would a more explicit error message be useful here?

[sgillies]

If you open a file in "r" mode and try to set the encoding attribute of the file object, you'll see AttributeError: readonly attribute. I think we could do something like that.

[spolloni]

@sgillies great! thanks for looking into this