Fix error handling with fallback in which implementation

Signed-off-by: Dan Ryan <dan@danryan.co>

pipenv/core.py

@@ -1524,22 +1524,30 @@ def system_which(command, mult=False):
         vistir.compat.fs_str(k): vistir.compat.fs_str(val)
         for k, val in os.environ.items()
     }
-    c = delegator.run("{0} {1}".format(_which, command))
     try:
-        # Which Not found…
-        if c.return_code == 127:
-            click.echo(
-                "{}: the {} system utility is required for Pipenv to find Python installations properly."
-                "\n  Please install it.".format(
-                    crayons.red("Warning", bold=True), crayons.red(_which)
-                ),
-                err=True,
-            )
-        assert c.return_code == 0
-    except AssertionError:
-        return None if not mult else []
-
-    result = c.out.strip() or c.err.strip()
+        c = delegator.run("{0} {1}".format(_which, command))
+        try:
+            # Which Not found…
+            if c.return_code == 127:
+                click.echo(
+                    "{}: the {} system utility is required for Pipenv to find Python installations properly."
+                    "\n  Please install it.".format(
+                        crayons.red("Warning", bold=True), crayons.red(_which)
+                    ),
+                    err=True,
+                )
+            assert c.return_code == 0
+        except AssertionError:
+            return None if not mult else[]
+    except TypeError:
+        from .vendor.pythonfinder import Finder
+        finder = Finder()
+        result = finder.which(command)
+        if result:
+            return result.path.as_posix()
+        return
+    else:
+        result = c.out.strip() or c.err.strip()
     if mult:
         return result.split("\n")