Add more informative error message in @import_required_backends

Failed in 4m 20s
๐Ÿ’ป test with incomplete environmentjulia --project -E "import ClimaComms, Test; @test_throws ErrorException ClimaComms.@import_required_backends"
Waited 29s
ยท
Ran in 3s
Total Job Run Time: 4m 41s