Make Sys.which return nothing instead of throwing

Currently `Sys.which("prog")` will throw an error if `prog` is not
found or is found but is not executable. This changes the function to
instead return `nothing` in those cases, thereby simplifying logic for
situations such as hierarchically choosing an executable based on which
in a list is present.

Fixes #27284.

base/sysinfo.jl

@@ -401,8 +401,8 @@ function which(program_name::String)
         end
     end
 
-    # If we couldn't find anything, complain
-    error("$program_name not found")
+    # If we couldn't find anything, don't return anything
+    nothing
 end
 which(program_name::AbstractString) = which(String(program_name))
 

test/spawn.jl

@@ -561,12 +561,12 @@ mktempdir() do dir
             @test Sys.which(foo_path) == realpath(foo_path)
 
             chmod(foo_path, 0o666)
-            @test_throws ErrorException Sys.which("foo")
-            @test_throws ErrorException Sys.which(foo_path)
+            @test Sys.which("foo") === nothing
+            @test Sys.which(foo_path) === nothing
         end
 
-        # Test that completely missing files also fail
-        @test_throws ErrorException Sys.which("this_is_not_a_command")
+        # Test that completely missing files also return nothing
+        @test Sys.which("this_is_not_a_command") === nothing
     end
 end