catch OSError to prevent breaking request /recon/diskusage

swift.common.utils.ismount maybe raise some OSError in some special
cases; and the request against /recon/diskusage doesn't handle it
before. This patch let output of mounted keyword is the error's message.

Change-Id: I5d9018f580181e618a3fa072b7a760d41795d8eb
Closes-Bug: #1249181