fix inconsistent i18n log message

An inconsistent example:
    imessage = _("Invalid metadata") + ": %(reason)s"
This is not consistent i18n usage. In most of cases, we use _() for
whole string instead of leaving something out.

More than inconsistent, some text in leaving strings may be not
translated, for example, ':' in English is different '：' in Chinese.

Change-Id: I00e690021c44f1debbe034cc0837a07b292ffd1b
Closes-Bug: #1220813
Merge "fix inconsistent i18n log message"