Improve MethodError message for anonymous functions (#57340)

If a MethodError arises on a anonyomous function, the words "anonymous
function" are printed in the error like so:
```julia
g=(x,y)->x+y
g(1,2,3)
```
```
ERROR: MethodError: no method of the anonymous function var"#5#6" matching (::var"#5#6")(::Int64, ::Int64, ::Int64)
The function `#5` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  (::var"#5#6")(::Any, ::Any)
   @ Main REPL[4]:1
```

See the [original pull
request](#57319) and
[issue](#56325) #56325

Author: Bumblebee00

base/errorshow.jl

@@ -346,7 +346,9 @@ function showerror(io::IO, ex::MethodError)
         curworld = get_world_counter()
         print(io, "\nThe applicable method may be too new: running in world age $(ex.world), while current world is $(curworld).")
     elseif f isa Function
-        print(io, "\nThe function `$f` exists, but no method is defined for this combination of argument types.")
+        print(io, "\nThe ")
+        isgensym(nameof(f)) && print(io, "anonymous ")
+        print(io, "function `$f` exists, but no method is defined for this combination of argument types.")
     elseif f isa Type
         print(io, "\nThe type `$f` exists, but no method is defined for this combination of argument types when trying to construct it.")
     else

test/errorshow.jl

@@ -1262,3 +1262,10 @@ end
 
 f33793(x::Float32, y::Float32) = 1
 @test_throws "\nClosest candidates are:\n  f33793(!Matched::Float32, !Matched::Float32)\n" f33793(Float64(0.0), Float64(0.0))
+
+# https://github.com/JuliaLang/julia/issues/56325
+let err_str
+    f56325 = x->x+1
+    err_str = @except_str f56325(1,2) MethodError
+    @test occursin("The anonymous function", err_str)
+end