Fix bug in IAllocator parsing of Evacuate result

Commit 342f9172 added stricter checks for the iallocator result in
evacuate mode, but it does this irrespective of the result
status. When the result has failed and (according to the design) the
list of nodes is empty, this code will trigger the following:

    node1# gnt-instance replace-disks -I hail instance14
    Failure: command execution error:
    Groups of nodes returned by iallocator () differ from original groups (default)

After the patch, the result is:

    node1# gnt-instance replace-disks -I hail instance14
    Failure: prerequisites not met for this operation:
    error type: insufficient_resources, error details:
    Can't compute nodes using iallocator 'hail': Request failed: …

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>