Better error report when src and dst datasets are the same

[Mateusz Łoskot]

Here is example of ogr2ogr use case in which a user specifies the same data
set for both: source and destination:

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ogr2ogr -s_srs EPSG:2180 -t_srs EPSG:2173 D:\OCK_region.shp D:\OCK_region.shp
ERROR 1: D:\OCK_region.shp is not a directory.
ESRI Shapefile driver failed to create D:\OCK_region.shp
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The error message is unclear and user may be confused because it's hard to find out what is the actual reason of the problem.

Enhancement proposal is to make this error message more meaningful.