fix instructions for building UI

"yarn build" doesn't actually fetch packages. When starting from a clean
client, you have to run "yarn" (with no arguments) first.

guide/install.md

@@ -84,6 +84,7 @@ For instructions, you can skip to "[Camera configuration and hard disk mounting]
 
 Once prerequisites are installed, Moonfire NVR can be built as follows:
 
+    $ yarn
     $ yarn build
     $ cargo test
     $ cargo build --release

prep.sh

@@ -187,10 +187,12 @@ if [ ! -d "${LIB_DIR}/ui" ]; then
 		echo
 		exit 1
 	fi
+	if ! yarn; then
+		echo "UI package installation failed."
+		exit 1
+	fi
 	if ! yarn build; then
 		echo "UI build failed."
-		echo "yarn build"
-		echo
 		exit 1
 	fi
 	sudo mkdir "${LIB_DIR}"