Implemented Exceptions for Unloaded Backends (#467)

* Updated error message when attempting to read a file without the appropriate backend.

The previous error message for reading rasters without an imported backend was confusing. The new message informs callers of the change and suggests the appropriate backend to use.

* Update sources.jl

* Updated error message when attempting to read a file without the appropriate backend.

The previous error message for reading rasters without an imported backend was confusing. The new message informs callers of the change and suggests the appropriate backend to use.

* Updated error message when attempting to read a file without the appropriate backend.

The previous error message for reading rasters without an imported backend was confusing. The new message informs callers of the change and suggests the appropriate backend to use.

The error now reads, "`Rasters.jl` requires backends to be loaded externally as of version 0.8. Run `import $packagename` to read $filename"

* Added warning to docs regarding extensions

* Added warning to docs regarding extensions

* Added warning to docs regarding extensions

* Implemented  to display a common error message when appropriate backend is not loaded

* Made BackendException non-mutable

README.md

@@ -16,7 +16,7 @@ multi-layered stacks, and multi-file series of arrays and stacks.
 
 _A RasterStack of EarthEnv HabitatHeterogeneity layers, trimmed to Australia and plotted with Plots.jl_
 
-## Packages extensions and Rasters 0.8 and onwards
+## :warning: Packages extensions and Rasters 0.8 and onwards 
 
 On Julia 1.9 we can put additional packages in extensions, so the code only loads when
 you load a specific package. Rasters.jl was always intended to work like this,

src/extensions.jl

@@ -1,8 +1,8 @@
 # extensions
 
 # stubs that need ArchGDAL
-resample(args...; kw...) = error("Run `using ArchGDAL` to use `resample`")
-warp(args...; kw...) = error("Run `using ArchGDAL` to use `warp`")
+resample(args...; kw...) = throw(BackendException("ArchGDAL"))
+warp(args...; kw...) = throw(BackendException("ArchGDAL"))
 
 # Other shared stubs
 function layerkeys end

src/sources/sources.jl

@@ -38,6 +38,16 @@ const EXT2SOURCE = Dict(
  ".h5" => SMAPsource
 )
 
+# exception to be raised when backend extension is not satisfied
+struct BackendException <: Exception
+ backend
+end
+
+# error message to show when backend is not loaded
+function Base.showerror(io::IO, e::BackendException)
+ print(io, "`Rasters.jl` requires backends to be loaded externally as of version 0.8. Run `import $(e.backend)` to fix this error.")
+end
+
 # Get the source backend for a file extension, falling back to GDALsource
 _sourcetype(filename::AbstractString) = get(EXT2SOURCE, splitext(filename)[2], GDALsource)
 _sourcetype(filenames::NamedTuple) = _sourcetype(first(filenames))
@@ -59,5 +69,5 @@ function _open(f, filename::AbstractString; source=_sourcetype(filename), kw...)
 end
 function _open(f, T::Type, filename::AbstractString; kw...)
  packagename = SOURCE2PACAKGENAME[T]
- error("`Rasters.jl` requires backends to be loaded externally as of version 0.8. Run `import $packagename` to read $filename")
+ throw(BackendException(packagename))
 end