Change non-unit error to assertion

Author: MilesCranmer

src/symbolic_dimensions.jl

@@ -114,7 +114,7 @@ Convert a quantity `q` with base SI units to the symbolic units of `qout`, for `
 Mathematically, the result has value `q / expand_units(qout)` and units `dimension(qout)`. 
 """
 function uconvert(qout::AbstractQuantity{<:Any, <:SymbolicDimensions}, q::AbstractQuantity{<:Any, <:Dimensions})
-    isone(ustrip(qout)) || error("You passed a quantity with a non-unit value to uconvert.")
+    @assert isone(ustrip(qout)) "You passed a quantity with a non-unit value to uconvert."
     qout_expanded = expand_units(qout)
     dimension(q) == dimension(qout_expanded) || throw(DimensionError(q, qout_expanded))
     new_val = ustrip(q) / ustrip(qout_expanded)

test/unittests.jl

@@ -604,7 +604,7 @@ end
     @test expand_units(qs) ≈ 5.0 * q
 
     # Refuses to convert to non-unit quantities:
-    @test_throws ErrorException uconvert(1.2us"m", 1.0u"m")
+    @test_throws AssertionError uconvert(1.2us"m", 1.0u"m")
     VERSION >= v"1.8" &&
         @test_throws "You passed a quantity" uconvert(1.2us"m", 1.0u"m")