Fix InexactError call in convert

[jishnub]

On master:

julia> convert(Float64, Dual(1,1))
ERROR: MethodError: no method matching InexactError()

After this

julia> convert(Float64, Dual(1,1))
ERROR: InexactError: convert(Float64, 1 + 1ɛ)

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (85bc6b2) 50.94% compared to head (dd954c7) 50.94%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #99   +/-   ##
=======================================
  Coverage   50.94%   50.94%           
=======================================
  Files           2        2           
  Lines         212      212           
=======================================
  Hits          108      108           
  Misses        104      104           

Files Changed	Coverage Δ
src/dual.jl	50.71% <100.00%> (ø)

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