impr: Show an error message when ImHex can't open a file (#1050)

This PR does two correlated things:

- Show a generic error message (Failed to open provider) rather than a
file-specific message (Failed to open file) when a provider fails to
open
- Set the error to something more specific when opening a file fails

plugins/builtin/source/content/welcome_screen.cpp

@@ -180,7 +180,7 @@ namespace hex::plugin::builtin {
             provider->loadSettings(recentProvider.data);
 
             if (!provider->open() || !provider->isAvailable()) {
-                PopupError::open("hex.builtin.popup.error.open"_lang);
+                PopupError::open(hex::format("hex.builtin.provider.error.open"_lang, provider->getErrorMessage()));
                 TaskManager::doLater([provider] { ImHexApi::Provider::remove(provider); });
                 return;
             }