Error message from @import_required_backends when CUDA.jl/MPI.jl are not available is unhelpful

[Sbozzolo]

When calling @import_required_backends, the relevant backends have to be installed. When they are not, the error message is something like

ERROR: ArgumentError: Package CUDA not found in current path.
- Run `import Pkg; Pkg.add("CUDA")` to install the CUDA package.
Stacktrace:
 [1] macro expansion
   @ ./loading.jl:1772 [inlined]
 [2] macro expansion
   @ ./lock.jl:267 [inlined]
 [3] __require(into::Module, mod::Symbol)
   @ Base ./loading.jl:1753
 [4] #invoke_in_world#3
   @ ./essentials.jl:926 [inlined]
 [5] invoke_in_world
   @ ./essentials.jl:923 [inlined]
 [6] require(into::Module, mod::Symbol)
   @ Base ./loading.jl:1746
 [7] macro expansion
   @ ~/.julia/packages/ClimaComms/agO9C/src/loading.jl:51 [inlined]
 [8] top-level scope
   @ REPL[6]:1

This is not very user friendly and might leave people confused.

This is something that end users interact with directly, so it would be better to provide more informative errors.