tweak docstrings, error message, and add test

src/machines.jl

@@ -1089,9 +1089,10 @@ function save(file::Union{String,IO}, mach::Machine)
 end
 
 const ERR_INVALID_DEFAULT_LOGGER = ArgumentError(
-    "`default_logger()` is currently `nothing`. "*
-    "Either specify an explicit path or stream as "*
-    "target of the save, or use `default_logger(logger)` "*
+    "You have attempted to save a machine to the default logger "*
+    "but `default_logger()` is currently `nothing`. "*
+    "Either specify an explicit logger, path or stream to save to, "*
+    "or use `default_logger(logger)` "*
     "to change the default logger. "
 )
 

src/resampling.jl

@@ -760,7 +760,7 @@ const DOC_DEFAULT_LOGGER =
 
    !!! note
 
-   In MLJ version prior to 0.21 the default logger is always `nothing`.
+   Prior to MLJ v0.20.7 (and MLJBase 1.5) the default logger was always `nothing`.
 
 """
 

test/resampling.jl

@@ -932,4 +932,21 @@ end
     end
 end
 
+# DUMMY LOGGER
+
+struct  DummyLogger end
+
+MLJBase.save(logger::DummyLogger, mach::Machine) = mach.model
+
+@testset "default logger" begin
+    @test isnothing(default_logger())
+    model = ConstantClassifier()
+    mach = machine(model, make_moons(10)...)
+    fit!(mach, verbosity=0)
+    @test_throws MLJBase.ERR_INVALID_DEFAULT_LOGGER MLJBase.save(mach)
+    default_logger(DummyLogger())
+    @test default_logger() == DummyLogger()
+    @test MLJBase.save(mach) == model
+end
+
 true