Move warning about plotlybase png rendering to error

[fonsp]

Before this PR, you would always see a warning about rendering to png when using the plotly backend.

Not so nice, and confusing for many users!

An easy improvement would be to change this to @info, but in this PR, I used the super powerful Base.Experimental.register_error_hint feature to move this "warning" to when it becomes relevant: when you try to render to png!

After this PR, the warning message is gone when calling plotly(), and it magically appears in the error message!

julia> using Plots

julia> plotly()
Plots.PlotlyBackend()

julia> p = plot(1:10);

julia> repr(MIME"image/png"(), p)
ERROR: MethodError: no method matching _show(::IOBuffer, ::MIME{Symbol("image/png")}, ::Plots.Plot{Plots.PlotlyBackend})

Tip: For saving/rendering as png with the `Plotly` backend `PlotlyBase` and `PlotlyKaleido` need to be installed.

Closest candidates are:
  _show(::IO, ::MIME{Symbol("image/png")}, ::Plots.Plot{Plots.GRBackend}) at ~/.julia/packages/Plots/etZYh/src/backends/gr.jl:2041
  _show(::IO, ::MIME{Symbol("application/vnd.plotly.v1+json")}, ::Plots.Plot{Plots.PlotlyBackend}) at ~/.julia/packages/Plots/etZYh/src/backends/plotly.jl:1131
  _show(::IO, ::MIME{Symbol("text/html")}, ::Plots.Plot{Plots.PlotlyBackend}) at ~/.julia/packages/Plots/etZYh/src/backends/plotly.jl:1134
  ...
Stacktrace:
...

julia> 

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.01 ⚠️

Comparison is base (97a72fd) 90.35% compared to head (2faa57d) 90.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4733      +/-   ##
==========================================
- Coverage   90.35%   90.34%   -0.01%     
==========================================
  Files          40       40              
  Lines        8731     8733       +2     
==========================================
+ Hits         7889     7890       +1     
- Misses        842      843       +1     

Impacted Files	Coverage Δ
src/backends.jl	82.26% <33.33%> (-0.33%)	⬇️

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[t-bltg]

Thanks, nicely explained !