catch stderr from yarn subprocess, addresses #2301

src/command-line/install.js

@@ -90,6 +90,10 @@ program
 				});
 			});
 
+			add.stderr.on("data", (data) => {
+				log.error(data.toString());
+			});
+
 			add.on("error", (e) => {
 				log.error(`${e}`);
 				process.exit(1);

src/command-line/uninstall.js

@@ -80,6 +80,10 @@ program
 			});
 		});
 
+		remove.stderr.on("data", (data) => {
+			log.error(data.toString());
+		});
+
 		remove.on("error", errorHandler);
 
 		remove.on("close", (code) => {