Add register_error_hint for UndefVarError (#39850)

* add show_error_hints for UndefVarError

Hi, I just include the show_error_hints part from #39089
Further relevant discussion in [discourse](https://discourse.julialang.org/t/help-with-register-error-hint/56140/4)

* Update errorshow.jl

* Update Base.jl

Co-authored-by: Simeon Schaub <simeondavidschaub99@gmail.com>

Author: josePereiro

base/Base.jl

@@ -337,6 +337,9 @@ include("meta.jl")
 include("stacktraces.jl")
 using .StackTraces
 
+# experimental API's
+include("experimental.jl")
+
 # utilities
 include("deepcopy.jl")
 include("download.jl")
@@ -360,9 +363,6 @@ include("util.jl")
 
 include("asyncmap.jl")
 
-# experimental API's
-include("experimental.jl")
-
 # deprecated functions
 include("deprecated.jl")
 

base/errorshow.jl

@@ -159,14 +159,18 @@ showerror(io::IO, ex::UndefKeywordError) =
     print(io, "UndefKeywordError: keyword argument $(ex.var) not assigned")
 
 function showerror(io::IO, ex::UndefVarError)
+    print(io, "UndefVarError: $(ex.var) not defined")
+    Experimental.show_error_hints(io, ex)
+end
+
+Experimental.register_error_hint(UndefVarError) do io::IO, ex::UndefVarError
     if ex.var in [:UTF16String, :UTF32String, :WString, :utf16, :utf32, :wstring, :RepString]
-        return showerror(io, ErrorException("""
-        `$(ex.var)` has been moved to the package LegacyStrings.jl:
-        Run Pkg.add("LegacyStrings") to install LegacyStrings on Julia v0.5-;
-        Then do `using LegacyStrings` to get `$(ex.var)`.
-        """))
+        println(io)
+        print(io, """
+            `$(ex.var)` has been moved to the package LegacyStrings.jl:
+            Run Pkg.add("LegacyStrings") to install LegacyStrings on Julia v0.5-;
+            Then do `using LegacyStrings` to get `$(ex.var)`.""")
     end
-    print(io, "UndefVarError: $(ex.var) not defined")
 end
 
 function showerror(io::IO, ex::InexactError)

test/errorshow.jl

@@ -797,3 +797,5 @@ if Sys.isapple() || (Sys.islinux() && Sys.ARCH === :x86_64)
         end
     end
 end  # Sys.isapple()
+
+@test contains(sprint(Base.showerror, UndefVarError(:UTF16String)), "LegacyStrings")