stage1: better file path handling

* better message printed when cache hash fails
 * better handling of '/' as root source file
 * os_path_split parses '/' and '/a' correctly

closes #1693
closes #1746

src/error.cpp

@@ -33,6 +33,7 @@ const char *err_str(Error err) {
         case ErrorSharingViolation: return "sharing violation";
         case ErrorPipeBusy: return "pipe busy";
         case ErrorPrimitiveTypeNotFound: return "primitive type not found";
+        case ErrorCacheUnavailable: return "cache unavailable";
     }
     return "(invalid error)";
 }