Misleading DimensionError message

[sostock]

In the following, conversion from a Time quantity to a Power*Time quantity fails (as it should), but the error message claims that the dimensions Time and Time are incompatible:

julia> convert(typeof(1.0u"dBm*s"), 1u"s")
ERROR: DimensionError: s and 1 s are not dimensionally compatible.
[…]

This is because the error message only considers the linear unit s, not the logarithmic dBm. This is due to using U() in the following line:

Unitful.jl/src/conversion.jl

Line 106 in b80ab85

throw(DimensionError(U(),x))