tools/gen-device-svd: show better error messages

Show an error message when input .svd files cannot be found.

tools/gen-device-svd.py

@@ -372,9 +372,16 @@ Default_Handler:
         out.write('    IRQ {name}_IRQHandler\n'.format(**intr))
 
 def generate(indir, outdir, sourceURL):
+    if not os.path.isdir(indir):
+        print('cannot find input directory:', indir, file=sys.stderr)
+        sys.exit(1)
     if not os.path.isdir(outdir):
         os.mkdir(outdir)
-    for filepath in sorted(glob(indir + '/*.svd')):
+    infiles = glob(indir + '/*.svd')
+    if not infiles:
+        print('no .svd files found:', indir, file=sys.stderr)
+        sys.exit(1)
+    for filepath in sorted(infiles):
         print(filepath)
         device = readSVD(filepath, sourceURL)
         writeGo(outdir, device)