Typecheck return value of chadrc; propagate errors

* Don't blindly assume the only way `require "custom/chadrc.lua" can
  fail is that the file doesn't exist.
* Provide user with a more useful message when return value is wrong

lua/core/utils.lua

@@ -141,12 +141,16 @@ end
 M.load_config = function()
    local conf = require "core.default_config"
 
-   local chadrcExists, change = pcall(require, "custom.chadrc")
-
-   -- if chadrc exists , then merge its table into the default config's
-
-   if chadrcExists then
-      conf = vim.tbl_deep_extend("force", conf, change)
+   -- attempt to load and merge a user config
+   local chadrc_exists = vim.fn.filereadable(vim.fn.stdpath "config" .. "/lua/custom/chadrc.lua") == 1
+   if chadrc_exists then
+      -- merge user config if it exists and is a table; otherwise display an error
+      local user_config = require "custom.chadrc"
+      if type(user_config) == 'table' then
+         conf = vim.tbl_deep_extend("force", conf, user_config)
+      else
+         error("User config (chadrc.lua) *must* return a table!")
+      end
    end
 
    return conf