fixed error message for cache proxy creation

[ar0ne]

What do these changes do?

See #794

Don't want to resolve merge conflicts between this branch and master.

Are there changes in behavior for the user?

Related issue number

Fixes #539

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a6a131e) 99.55% compared to head (6c39533) 99.55%.

Additional details and impacted files

@@           Coverage Diff           @@
##             0.12     #795   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          35       35           
  Lines        3782     3783    +1     
=======================================
+ Hits         3765     3766    +1     
  Misses         17       17           

Flag	Coverage Δ
unit	99.55% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
aiocache/factory.py	100.00% <100.00%> (ø)
tests/ut/test_factory.py	97.64% <100.00%> (+0.01%)	⬆️

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