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/ruby_spec.lua

@@ -19,8 +19,9 @@ do
   clear()
   if missing_provider('ruby') then
     it(':ruby reports E319 if provider is missing', function()
-      expect_err([[Vim%(ruby%):E319: No "ruby" provider found.*]],
-      command, 'ruby puts "foo"')
+      local expected = [[Vim%(ruby.*%):E319: No "ruby" provider found.*]]
+      expect_err(expected, command, 'ruby puts "foo"')
+      expect_err(expected, command, 'rubyfile foo')
     end)
     pending("Missing neovim RubyGem.", function() end)
     return