Raise an error if adjoint is applied to a non-callable object.

[dwierichs]

Context:
This implements #1336

Description of the Change:
The object that qml.adjoint is applied to is checked to be callable before constructing the transform.

Benefits:
Catches scenarios in which qml.adjoint would silently simply do nothing.

Possible Drawbacks:
n/a

Related GitHub Issues:
closes #1336

[codecov]

Codecov Report

Merging #2060 (e6f1be0) into master (7092213) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2060   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files         226      226           
  Lines       17368    17370    +2     
=======================================
+ Hits        17226    17229    +3     
+ Misses        142      141    -1     

Impacted Files	Coverage Δ
pennylane/transforms/adjoint.py	100.00% <100.00%> (ø)
pennylane/tape/tape.py	99.21% <0.00%> (+0.26%)	⬆️

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[josh146]

Thanks @dwierichs!