util: Adjust error paths for virNumaSetPagePoolSize

util: Adjust error paths for virNumaSetPagePoolSize

Commit id '1c24cfe9' added new checks and error messaes for failure
scenarios. Let's adjust those error messages to after the call to
virNumaGetHugePageInfoPath in order to provide a more specific error
message depending on node and page_size

After this patch:
 # virsh allocpages --pagesize 2047 --pagecount 1 --cellno 0
 error: operation failed: page size 2047 is not available on node 0

 # virsh allocpages --pagesize 2047 --pagecount 1
 error: operation failed: page size 2047 is not available

Signed-off-by: Luyao Huang <lhuang@redhat.com>