tools: improve error message when --ui is passed on Windows

Change-Id: Ia52dfed94615fd00c6cb54a0b8e25c1117b2b89b
diff --git a/tools/install-build-deps b/tools/install-build-deps
index 835ee9c..9e3305a 100755
--- a/tools/install-build-deps
+++ b/tools/install-build-deps
@@ -484,6 +484,12 @@
   if args.verify:
     CheckHashes()
     return 0
+
+  target_os = system().lower()
+  if args.ui and target_os == 'windows':
+    print('Building the UI on Windows is unsupported')
+    return 1
+
   deps = BUILD_DEPS_HOST
   if not args.no_toolchain:
     deps += BUILD_DEPS_TOOLCHAIN_HOST
@@ -498,7 +504,6 @@
     RmtreeIfExists(os.path.join(ROOT_DIR, old_dir))
 
   for dep in deps:
-    target_os = system().lower()
     target_arch = GetArch()
     matches_os = dep.target_os == 'all' or target_os == dep.target_os
     matches_arch = dep.target_arch == 'all' or target_arch == dep.target_arch