provider: improve error message if provider is missing (#9487)

Move `has_eval_provider()` check to `eval_call_provider()` to make sure that
every code path calls it first.

Previously we would, when pynvim was missing, get a nice error message for
`:python3 1`, but not for `:py3file blah`.

Fixes https://github.com/neovim/neovim/issues/9485

test/functional/provider/python_spec.lua

@@ -19,8 +19,9 @@ do
   clear()
   if missing_provider('python') then
     it(':python reports E319 if provider is missing', function()
-      expect_err([[Vim%(python%):E319: No "python" provider found.*]],
-                 command, 'python print("foo")')
+      local expected = [[Vim%(py.*%):E319: No "python" provider found.*]]
+      expect_err(expected, command, 'py print("foo")')
+      expect_err(expected, command, 'pyfile foo')
     end)
     pending('Python 2 (or the pynvim module) is broken/missing', function() end)
     return