Use error hint instead of explicit error

The latter will lead to invalidations when JLD2 is loaded

Author: simsurace

src/MetaGraphs.jl

@@ -581,4 +581,20 @@ include("metadigraph.jl")
 include("metagraph.jl")
 include("persistence.jl")
 include("overrides.jl")
+
+function __init__()
+    # Register error hint for the `loadmg`, `savemg`, and `savedot` functions
+    if isdefined(Base.Experimental, :register_error_hint)
+        Base.Experimental.register_error_hint(MethodError) do io, exc, _, _
+            if exc.f === loadsg
+                print(io, "\n\nIn order to load meta graphs from binary files, you need \
+                to load the JLD2.jl package.")
+            elseif exc.f === savesg
+                print(io,"\n\nIn order to save meta graphs to binary files, you need to \
+                load the JLD2.jl package.")
+            end
+        end
+    end
+end
+
 end # module

src/persistence.jl

@@ -3,20 +3,9 @@
 struct MGFormat <: AbstractGraphFormat end
 struct DOTFormat <: AbstractGraphFormat end
 
-function loadmg(args...)
-    error("In order to load static graphs from binary files, you need to load the JLD2.jl \
-    package")
-end
-
-function savemg(args...)
-    error("In order to save static graphs to binary files, you need to load the JLD2.jl \
-    package")
-end
-
-function savedot(args...)
-    error("In order to save static graphs to binary files, you need to load the JLD2.jl \
-    package")
-end
+function loadmg end
+function savemg end
+function savedot end
 
 loadgraph(fn::AbstractString, ::String, ::MGFormat) = loadmg(fn)
 savegraph(fn::AbstractString, g::AbstractMetaGraph) = savemg(fn, g)