readable DimensionError message

[ggggggggg]

It's hard to read the types of long units, so I put a more readable message in for DimensionError. With this PR

julia> 1u"m"+1u"s"
ERROR: DimensionError: 1 m and 1 s are not dimensionally compatible.
 in +(::Unitful.Quantity{Int64,Unitful.Dimensions{(Unitful.Dimension{:Length}(1//1),)},Unitful.Units{(Unitful.Unit{:Meter,Unitful.Dimensions{(Unitful.Dimension{:Length}(1//1),)}}(0,1//1),),Unitful.Dimensions{(Unitful.Dimension{:Length}(1//1),)}}}, ::Unitful.Quantity{Int64,Unitful.Dimensions{(Unitful.Dimension{:Time}(1//1),)},Unitful.Units{(Unitful.Unit{:Second,Unitful.Dimensions{(Unitful.Dimension{:Time}(1//1),)}}(0,1//1),),Unitful.Dimensions{(Unitful.Dimension{:Time}(1//1),)}}}) at /Users/oneilg/.julia/v0.5/Unitful/src/Unitful.jl:412

I'd suggest a reading over Conversion.jl and Promotion.jl fairly close, as I had few decisions about what to do with ::Type arguments, and you may prefer something else.

[ajkeller34]

Looks good, thanks! I made some small changes and will merge this after CI passes.

[coveralls]

Coverage increased (+0.03%) to 67.91% when pulling 1dac617 on ggggggggg:pull-request/77313054 into b39ac60 on ajkeller34:master.