remove UndefinedFunctionError requirement for logging missing source error

[sergey-ivlev]

log error instead of crashes like this

** (FunctionClauseError) no function clause matching in IO.chardata_to_string/1    
    
    The following arguments were given to IO.chardata_to_string/1:
    
        # 1
        nil
    
    Attempted function clauses (showing 2 out of 2):
    
        def chardata_to_string(+string+) when -is_binary(string)-
        def chardata_to_string(+list+) when -is_list(list)-
    
    (elixir) lib/io.ex:557: IO.chardata_to_string/1
    (elixir) lib/file.ex:214: File.exists?/2
    (excoveralls) lib/excoveralls/cover.ex:43: ExCoveralls.Cover.has_compile_info?/1
    (elixir) lib/enum.ex:2942: Enum.filter_list/2
    (elixir) lib/enum.ex:2943: Enum.filter_list/2
    (excoveralls) lib/excoveralls.ex:39: ExCoveralls.execute/2
    (mix) lib/mix/tasks/test.ex:446: Mix.Tasks.Test.do_run/3
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (excoveralls) lib/mix/tasks.ex:54: Mix.Tasks.Coveralls.do_run/2
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2
    (elixir) lib/code.ex:813: Code.require_file/2

[coveralls]

Coverage remained the same at 89.604% when pulling 357f305 on sergey-ivlev:nil_source into 864a73d on parroty:master.

[parroty]

Thanks!