DSLSchema transform type attribute assert into AttributeError

[leszekhanusz]

Better error for DSLSchema invalid attributes.
Changing an assert into an AttributeError.

See PR #406

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8e1c6f1) 100.00% compared to head (c6d0297) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #409   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         2384      2385    +1     
=========================================
+ Hits          2384      2385    +1     

Impacted Files	Coverage Δ
gql/dsl.py	100.00% <100.00%> (ø)

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