Only print a single LoadError: prefix for nested LoadErrors during showerror (#38725)

* fold repeated `LoadError :` prints during showerror

* Only print a single LoadError: prefix

Author: IanButterworth

base/errorshow.jl

@@ -92,7 +92,7 @@ function showerror(io::IO, ex, bt; backtrace=true)
 end
 
 function showerror(io::IO, ex::LoadError, bt; backtrace=true)
-    print(io, "LoadError: ")
+    !isa(ex.error, LoadError) && print(io, "LoadError: ")
     showerror(io, ex.error, bt, backtrace=backtrace)
     print(io, "\nin expression starting at $(ex.file):$(ex.line)")
 end