fix stacktrace handling on nightly

[KristofferC]

Before:

Expression: save("test.not_installed", nothing)
    Expected: CapturedException
      Thrown: MethodError
      MethodError: no method matching 
stacktrace(::Vector{Base.StackTraces.StackFrame})
      The function `stacktrace` exists, but no method is defined for 
this combination of argument types.
      Closest candidates are:
        stacktrace(::Bool)
         @ Base stacktraces.jl:206
        stacktrace()
         @ Base stacktraces.jl:206
        stacktrace(::Vector{<:Union{Ptr{Nothing}, Base.InterpreterIP}},
 ::Bool)
         @ Base stacktraces.jl:193
        ...
      Stacktrace:
        [1] CapturedException(ex::ArgumentError, 
bt_raw::Vector{Base.StackTraces.StackFrame})
          @ Base ./task.jl:17
        [2] handle_error(e::ArgumentError, q::Base.PkgId, 
bt::Vector{Union{Ptr{Nothing}, Base.InterpreterIP}})
          @ FileIO 
~/PkgEval/problemos/FileIO.jl/src/error_handling.jl:71
        [3] handle_exceptions(exceptions::Vector{Tuple{Any, 
Union{Base.PkgId, Module}, Vector}}, action::String)
          @ FileIO 
~/PkgEval/problemos/FileIO.jl/src/error_handling.jl:66
        [4] action(call::Symbol, libraries::Vector{Union{Base.PkgId, 
Module}}, file::Formatted, args::Nothing; options::@Kwargs{})
          @ FileIO ~/PkgEval/problemos/FileIO.jl/src/loadsave.jl:228
        [5] action
          @ ~/PkgEval/problemos/FileIO.jl/src/loadsave.jl:196 [inlined]
        [6] action
          @ ~/PkgEval/problemos/FileIO.jl/src/loadsave.jl:185 [inlined]
        [7] save(file::String, args::Nothing; options::@Kwargs{})
          @ FileIO ~/PkgEval/problemos/FileIO.jl/src/loadsave.jl:129
        [8] save(file::String, args::Nothing)
          @ FileIO ~/PkgEval/problemos/FileIO.jl/src/loadsave.jl:125

Not sure how the trim! functionality should be restored. I'll open an issue about that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (83af3dd) to head (fabe01d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
+ Coverage   91.40%   91.44%   +0.03%     
==========================================
  Files          11       11              
  Lines         710      713       +3     
==========================================
+ Hits          649      652       +3     
  Misses         61       61              

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