lvm2app: Reset buffer after retrieving error message

lvm2app: Reset buffer after retrieving error message

The error buffer will stack error messages which is fine.  However,
once you retrieve the error messages it doesn't make sense to keep
appending for each additional error message when running in the
context of a library call.

This patch clears and resets the buffer after the user retrieves
the error message.

Signed-off-by: Tony Asleson <tasleson@redhat.com>