Cleanup error in servedocs when a page is not there

[tlienart]

Here's a stacktrace, the make.jl was read before a page was found; probably shouldn't error but just throw a warning message or something.

[ Info: SetupBuildDirectory: setting up build directory.
┌ Error: An error happened whilst watching files; shutting down. Error was: LoadError("/Users/tlienart/.julia/dev/JuDoc/docs/make.jl", 3, ErrorException("'man/troubleshooting.md' is not an existing page!"))
└ @ LiveServer ~/.julia/dev/LiveServer.jl/src/file_watching.jl:106
┌ Error: Exception while generating log record in module LiveServer at /Users/tlienart/.julia/dev/LiveServer.jl/src/file_watching.jl:106
│   exception =
│    InterruptException:
│    Stacktrace:
│     [1] try_yieldto(::typeof(Base.ensure_rescheduled), ::Base.RefValue{Task}) at ./task.jl:517
│     [2] wait() at ./task.jl:592
│     [3] uv_write(::Base.TTY, ::Ptr{UInt8}, ::UInt64) at ./stream.jl:883
│     [4] unsafe_write(::Base.TTY, ::Ptr{UInt8}, ::UInt64) at ./stream.jl:931
│     [5] unsafe_write at ./io.jl:522 [inlined]
│     [6] macro expansion at ./gcutils.jl:87 [inlined]
│     [7] write(::Base.TTY, ::Array{UInt8,1}) at ./io.jl:545
│     [8] #handle_message#2(::Nothing, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(Base.CoreLogging.handle_message), ::Logging.ConsoleLogger, ::Base.CoreLogging.LogLevel, ::String, ::Module, ::String, ::Symbol, ::String, ::Int64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.3/Logging/src/ConsoleLogger.jl:161
│     [9] handle_message(::Logging.ConsoleLogger, ::Base.CoreLogging.LogLevel, ::String, ::Module, ::String, ::Symbol, ::String, ::Int64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.3/Logging/src/ConsoleLogger.jl:100
│     [10] #handle_message#1(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(Base.CoreLogging.handle_message), ::Atom.Progress.JunoProgressLogger, ::Base.CoreLogging.LogLevel, ::String, ::Module, ::String, ::Symbol, ::String, ::Int64) at /Users/tlienart/.julia/packages/Atom/cR6bU/src/progress.jl:66
│     [11] handle_message(::Atom.Progress.JunoProgressLogger, ::Base.CoreLogging.LogLevel, ::String, ::Module, ::String, ::Symbol, ::String, ::Int64) at /Users/tlienart/.julia/packages/Atom/cR6bU/src/progress.jl:32
│     [12] macro expansion at ./logging.jl:320 [inlined]
│     [13] file_watcher_task!(::LiveServer.SimpleWatcher) at /Users/tlienart/.julia/dev/LiveServer.jl/src/file_watching.jl:106
│     [14] (::getfield(LiveServer, Symbol("##2#3")){LiveServer.SimpleWatcher})() at ./task.jl:268
└ @ LiveServer ~/.julia/dev/LiveServer.jl/src/file_watching.jl:106