Improve error message when helper package managers are not available

[glennmatthews]

$ cot inject-config csr1000v-universalk9.16.03.01a-w-ios-config.ova -o csr1000v-universalk9.16.03.01a-w-1Int-BaseConfig-ios-config.ova -c csr1000v-Config.txt
    INFO: Loading 'csr1000v-universalk9.16.03.01a-w-ios-config.ova' as OVF
    INFO: OVF version is 1.x
    INFO: OVF product class com.cisco.csr1000v --> platform CSR1000V
No helper available or installable!

This is failing because none of the ISO helpers (mkisofs/genisoimage/xorriso) are available and the user is on a Mac that does not have MacPorts installed - but the error message fails to make that clear, and should be improved.

[glennmatthews]

Improved messages:

>>> helper_select(['mkisofs', 'genisoimage', 'xorriso'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "COT/helpers/helper.py", line 655, in helper_select
    raise HelperNotFoundError(msg)
COT.helpers.helper.HelperNotFoundError: No helper in list ['mkisofs', 'genisoimage', 'xorriso'] is available or installable!
Unsure how to install mkisofs.
COT can use MacPorts (https://www.macports.org/), if available on your system, to install mkisofs and other helpers for you.
Unsure how to install genisoimage.
Unsure how to install xorriso.
>>> helper_select([('qemu-img', '2.0.0'), 'vmdktool'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "COT/helpers/helper.py", line 655, in helper_select
    raise HelperNotFoundError(msg)
COT.helpers.helper.HelperNotFoundError: No helper in list [('qemu-img', '2.0.0'), 'vmdktool'] is available or installable!
Unsure how to install qemu-img.
Refer to http://www.qemu.org for information
COT can use MacPorts (https://www.macports.org/), if available on your system, to install qemu-img and other helpers for you.
Unsure how to install vmdktool.
Refer to http://www.freshports.org/sysutils/vmdktool/ for information
>>>

>>> import sys
>>> sys.platform = 'linux2'
>>> helper_select(['mkisofs', 'genisoimage', 'xorriso'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "COT/helpers/helper.py", line 655, in helper_select
    raise HelperNotFoundError(msg)
COT.helpers.helper.HelperNotFoundError: No helper in list ['mkisofs', 'genisoimage', 'xorriso'] is available or installable!
Unsure how to install mkisofs.
Unsure how to install genisoimage.
COT can use package managers 'yum' or 'apt-get' to install helpers on your system, but it appears that you have neither of these package managers?
Unsure how to install xorriso.
COT can use package managers 'yum' or 'apt-get' to install helpers on your system, but it appears that you have neither of these package managers?

>>> sys.platform = 'windows'
>>> helper_select(['mkisofs', 'genisoimage', 'xorriso'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "COT/helpers/helper.py", line 655, in helper_select
    raise HelperNotFoundError(msg)
COT.helpers.helper.HelperNotFoundError: No helper in list ['mkisofs', 'genisoimage', 'xorriso'] is available or installable!
Unsure how to install mkisofs.
Unsure how to install genisoimage.
Unsure how to install xorriso.