Description of problem (please be detailed as possible and provide log
snippests):
When creating a Kubernetes Pool via the NooBaa web UI, there is no limit on the amount of pods the user can create. 

When creating a pv-store via the CLI, the limit is 20, but this is not communicated properly:
> nb backingstore create pv-pool testing --num-volumes=21
INFO[0002] ✅ Exists: NooBaa "noobaa"                    
FATA[0002] ❌ Number of volumes seems to be too large 21 

A good example of handling this can be seen when the user tries to use a size lower than 16GBs:
FATA[0007] ❌ PV size seems to be too small (15GB), minimal size for a pv is 16GB 

It states what was the user input, and what is the minimum

Version of all relevant components (if applicable):
NB 
5.3.0-df3abc2
OCS
4.3.0-376.ci

Does this issue impact your ability to continue to work with the product
(please explain in detail what is the user impact)?
No

Is there any workaround available to the best of your knowledge?
Create no more than 20 storage nodes

Rate from 1 - 5 the complexity of the scenario you performed that caused this
bug (1 - very simple, 5 - very complex)?
1


Actual results:
In the UI - creation is possible and goes through
In the CLI - the max amount isn't documented

Expected results:
In the UI - creation is blocked
In the CLI - the max amount is stated as part of the error message

Additional info:
I think it might be a good idea to include the maximum sizes in the options docs.
i.e.:
Options:
      --num-volumes=0: Number of volumes in the store (Max 20)
      --pv-size-gb=0: PV size of each volume in the store (Min. 16)
      --storage-class='': The storage class to use for PV provisioning