Show error message if curl is not installed (#58)

* handled launch process failed error

* stated curl as dependency in README

* fixed intentional misspelt curl

README.md

@@ -13,6 +13,9 @@ https://github.com/chomosuke/typst-preview.nvim/assets/38484873/9f8ecf0f-aa1c-4e
 
 ## 📦 Installation
 
+#### Dependencies
+- curl
+
 **Lazy.nvim:**
 
 ```lua

lua/typst-preview/fetch.lua

@@ -126,11 +126,8 @@ local function download_bin(bin, callback)
   local stdin = nil
   local stdout = assert(vim.uv.new_pipe())
   local stderr = assert(vim.uv.new_pipe())
-  -- TODO add wget support
-  vim.uv.spawn('curl', {
-    args = { '-L', url, '--create-dirs', '--output', path, '--progress-bar' },
-    stdio = { stdin, stdout, stderr },
-  }, function(code, _)
+
+  local function after_curl(code)
     if code ~= 0 then
       utils.notify(
         'Downloading ' .. name .. ' binary failed, exit code: ' .. code
@@ -143,7 +140,22 @@ local function download_bin(bin, callback)
         callback()
       end
     end
-  end)
+  end
+
+  -- TODO add wget support
+  local handle, err = vim.uv.spawn('curl', {
+    args = { '-L', url, '--create-dirs', '--output', path, '--progress-bar' },
+    stdio = { stdin, stdout, stderr },
+  }, after_curl)
+
+  if handle == nil then
+    utils.notify(
+      'Launching curl failed: '
+        .. err
+        .. '\nMake sure curl is installed on the system.'
+    )
+  end
+
   local function read_progress(err, data)
     if err then
       error(err)