avoid suggesting a method that will not work to resolve a method ambiguity

[JeffBezanson]

Example:

julia> f33789(x::I, ft::Function, use::Type{U}, ignore::Type{I}) where {U, I} = 0;

julia> f33789(x::U, ft::Function, use::Type{U}, ignore::Type{I}) where {U, I} = 1;

julia> f33789(1.0f0, identity, Number, Float32)
ERROR: MethodError: f33789(::Float32, ::typeof(identity), ::Type{Number}, ::Type{Float32}) is ambiguous. Candidates:
  f33789(x::I, ft::Function, use::Type{U}, ignore::Type{I}) where {U, I} in Main at REPL[1]:1
  f33789(x::U, ft::Function, use::Type{U}, ignore::Type{I}) where {U, I} in Main at REPL[2]:1
To resolve the ambiguity, try making one or more of the methods more specific, or adding a new method more specific than any of the existing applicable methods.

[aminya]

So this PR makes Julia give a better error message in case of the method ambiguity. The issue's title is a bit misleading.