Fix string joining in compiledump

s/list.join(s)/s.join(list)
and give the actual compiler (or None) that we can't find

Change-Id: I4a5e93a5465135f6a4dd16b39e79f7c93bff0756
diff --git a/tools/compiledump.py b/tools/compiledump.py
index 6f78eef..8403ca5 100755
--- a/tools/compiledump.py
+++ b/tools/compiledump.py
@@ -25,7 +25,7 @@
   parser.add_argument(
     '-c',
     '--compiler',
-    help='Compiler to use (default read from version file)',
+    help='Compiler to use',
     default=None)
   parser.add_argument(
     '-v',
@@ -103,9 +103,10 @@
   return args.version
 
 def determine_compiler(args, dump):
-  compilers = ('d8', 'r8', 'r8full')
+  compilers = ['d8', 'r8', 'r8full']
   if args.compiler not in compilers:
-    error("Unable to determine a compiler to use. Valid options: %s" % compilers.join(', '))
+    error("Unable to determine a compiler to use. Specified %s,"
+          " Valid options: %s" % (args.compiler, ', '.join(compilers)))
   return args.compiler
 
 def determine_output(args, temp):