Error handling for entr when watching multiple files

[ffevotte]

When watching multiple files with entr, and the user-provided function throws an error, entr does not immediately exit as I think it is intended to.

Steps to reproduce:

julia> import Revise
julia> import Dates

# Create two empty files
julia> open((f)->nothing, "/tmp/file1", "w")
julia> open((f)->nothing, "/tmp/file2", "w")

#... and watch them
julia> Revise.entr(["/tmp/file1", "/tmp/file2"]; postpone=true) do
           println("Something changed: ", Dates.now())
           error("KO")
       end

# Touch file1 from outside julia
# -> the user-provided function runs
Something changed: 2020-05-16T13:13:37.046
# -> the error has been thrown, but nothing seems to happen yet
# -> entr does not return

# Touch file1 again
# -> nothing happens; the user-provided function is not called

# Later touch file2
Something changed: 2020-05-16T13:14:00.926
# -> now the errors are shown and `entr` exits
ERROR: TaskFailedException:
KO
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] (::var"#9#10")() at ./REPL[5]:3
 [3] #invokelatest#1 at ./essentials.jl:712 [inlined]
 [4] invokelatest at ./essentials.jl:711 [inlined]
 [5] macro expansion at /home/francois/.julia/packages/Revise/AMRie/src/Revise.jl:828 [inlined]
 [6] (::Revise.var"#81#82"{Float64,var"#9#10",typeof(FileWatching.watch_file),String})() at ./task.jl:358

...and 1 more exception(s).

I think this happens because one @async task is spawned for each watched file, and the try...catch block handling exception surrounds an @sync block synchronizing all these tasks. So that when an error is thrown in one of the @async tasks, nothing happens until all other tasks terminate one way or another.

I'm working on a (tentative) fix, and should be able to propose a PR in the coming hours...

[ffevotte]

Closed by #488