Add uppercase first word warning.

Author: baizmandesign

features/plugin-list.feature

@@ -58,7 +58,7 @@ Feature: List WordPress plugins
     When I try `wp plugin list --skip-update-check --force-check`
     Then STDERR should contain:
       """
-      Error: plugin updates cannot be both force-checked and skipped. Choose one.
+      Error: Plugin updates cannot be both force-checked and skipped. Choose one.
       """
     And STDOUT should be empty
     And the return code should be 1

features/theme-list.feature

@@ -56,7 +56,7 @@ Feature: List WordPress themes
     When I try `wp theme list --skip-update-check --force-check`
     Then STDERR should contain:
       """
-      Error: theme updates cannot be both force-checked and skipped. Choose one.
+      Error: Theme updates cannot be both force-checked and skipped. Choose one.
       """
     And STDOUT should be empty
     And the return code should be 1

src/WP_CLI/CommandWithUpgrade.php

@@ -534,7 +534,7 @@ protected function _list( $_, $assoc_args ) {
 
 		// If `--force-check` and `--skip-update-check` flags are both present, abort.
 		if ( true === (bool) Utils\get_flag_value( $assoc_args, 'force-check', false ) && true === (bool) Utils\get_flag_value( $assoc_args, 'skip-update-check', false ) ) {
-			WP_CLI::error( "{$this->item_type} updates cannot be both force-checked and skipped. Choose one." );
+			WP_CLI::error( ucfirst( "{$this->item_type} updates cannot be both force-checked and skipped. Choose one." ) );
 		}
 
 		// If `--force-check` flag is present, delete the upgrade transient.