Description of problem:
When user specifies directory to cert-show command instead of file name,
command raises stack trace.

[root@vm-idm-010 ~]# ipa cert-show --certificate-out=/tmp
Serial number: 1
ipa: ERROR: non-public: IOError: [Errno 21] Is a directory: u'/tmp'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ipalib/backend.py", line 139, in
execute
    result = self.Command[_name](*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 447, in
__call__
    return self.__do_call(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 475, in
__do_call
    ret = self.run(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 798, in run
    return self.forward(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipaclient/plugins/cert.py", line 96,
in forward
    return super(cert_show, self).forward(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipaclient/plugins/cert.py", line 59,
in forward
    with open(certificate_out, 'w') as f:
IOError: [Errno 21] Is a directory: u'/tmp'
ipa: ERROR: an internal error has occurred


Version-Release number of selected component (if applicable):
# rpm -qa ipa-server
ipa-server-4.5.0-5.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. ipa cert-show 1 --certificate-out=/tmp

Actual results:
Stack trace with IOError

Expected results:
Notify user with error/warn message rather than stack trace.

"Directory specified instead of file name"