qapi: Improve 'include' error message

Use of '"...%s" % include' to print non-strings can lead to
ugly messages, such as this (if the .json change is applied
without the qapi.py change):
 Expected a file name (string), got: OrderedDict()

Better is to just omit the actual non-string value in the
message.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1443565276-4535-3-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 06478bb..362e007 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -132,8 +132,7 @@
                 include = expr["include"]
                 if not isinstance(include, str):
                     raise QAPIExprError(expr_info,
-                                        'Expected a file name (string), got: %s'
-                                        % include)
+                                        "Value of 'include' must be a string")
                 incl_abs_fname = os.path.join(os.path.dirname(abs_fname),
                                               include)
                 # catch inclusion cycle