GMP: fix warning in init

[vtjnash]

The warn function does not exist, and error has no effect here (it is wrapped in try/catch)

julia> @eval Base.GMP VERSION = v"1.2.3"
WARNING: redefinition of constant VERSION. This may fail, cause incorrect answers, or produce other errors.
v"1.2.3"

julia> @eval Base.GMP BITS_PER_LIMB = 32
WARNING: redefinition of constant BITS_PER_LIMB. This may fail, cause incorrect answers, or produce other errors.
32

julia> Base.GMP.__init__()
┌ Error: The dynamically loaded GMP library (v"6.2.1" with __gmp_bits_per_limb == 64)
│ does not correspond to the compile time version (v"1.2.3" with __gmp_bits_per_limb == 32).
│ Please rebuild Julia.
└ @ Base.GMP /data/vtjnash/julia1/base/gmp.jl:100
true

julia> @edit Base.GMP.__init__()

julia> Base.GMP.__init__()
WARNING: Error during initialization of module GMP:
ErrorException("could not load library "xlibgmp"
xlibgmp.so: cannot open shared object file: No such file or directory")
true