Better error message on `mv(x, x)`

[LilithHafner]

The error message when moving a file to it's current location could be better:

julia> mv("file", "file")
ERROR: ArgumentError: 'file' exists. `force=true` is required to remove 'file' before moving.
Stacktrace:
 [1] checkfor_mv_cp_cptree(src::String, dst::String, txt::String; force::Bool)
   @ Base.Filesystem ./file.jl:334
 [2] checkfor_mv_cp_cptree
   @ ./file.jl:318 [inlined]
 [3] #mv#15
   @ ./file.jl:427 [inlined]
 [4] mv(src::String, dst::String)
   @ Base.Filesystem ./file.jl:426
 [5] top-level scope
   @ REPL[93]:1