Extender: Return error if no replicas are online for a volume

[disrani-px]

What type of PR is this?
Bug (Related to #291)

What this PR does / why we need it:
If there are no replicas online for a PVC the pod should not get scheduled, since it will not be able to mount the volume anyways. This change returns an error from the Filter request to prevent scheduling such pods till atleast one node with the replica comes back online

Does this PR change a user-facing CRD or CLI?:

No

Is a release note needed?:

Extender will now return an error if no replicas for a volume are online. This will prevent pods from getting scheduled if they can not use a PVC.

Does this change need to be cherry-picked to a release branch?:
2.1

[codecov]

Codecov Report

Merging #299 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
+ Coverage   73.45%   73.65%   +0.19%     
==========================================
  Files          19       19              
  Lines        1379     1389      +10     
==========================================
+ Hits         1013     1023      +10     
  Misses        282      282              
  Partials       84       84

Impacted Files	Coverage Δ
pkg/extender/extender.go	85.16% <100%> (+0.74%)	⬆️

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