[gdal-dev] Write overviews directly to S3

[Even Rouault]

> 
> Even - I found methods to check for VSI errors:
> https://gist.github.com/pedros007/55c6e33224596fb4d8e9e6b68b24ed9b#file-simp
> levsimemfile-py-L73-L74
> 
> Are these intended for public consumption? 

With care...

> When reading from a
> /vsimem/foo.shp with gdal-2.2.0, gdal.VSIGetLastErrorMsg() reported an
> error "No such file or directory". 

Probably because the shapefile driver probed a side-car file that didn't exist (the .sbn you 
mention below) but wasn't required. VSIGetLastErrorMsg() is mostly for internal use by 
GDALOpen() (at least it was designed for that). Users will normally call 
gdal.GetLastErrorMsg() or gdal.UseExceptions() to have exceptions for user-visible errors.

Rule of thumb:
- gdal.VSIGetLastErrorMsg() can be used if you use low-level API like gdal.VSIFOpenL() that 
aren't verbose normally.
- for higher level like gdal.Open(), use gdal.GetLastErrorMsg() or gdal.UseExceptions()

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