Fixed an issue where backtrace handling of exception classes that return nil message fails.

[masato-hi]

Raise a new TypeError when handling an exception class that returns an nil message.
Changed to always treat exception message as String.

The actual case is as follows.

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'ddtrace', '0.49.0', require: 'ddtrace'
  gem 'thrift', require: 'thrift'
end


class SomethingException < Thrift::Exception
  include Thrift::Struct
  FIELDS = {}
  def struct_fields; FIELDS; end
  Thrift::Struct.generate_accessors self
end

begin
  Datadog.tracer.trace('SomethingException') do
    raise SomethingException
  end
rescue SomethingException => e
  p 'nice catch!', e.message
end

[codecov-commenter]

Codecov Report

Merging #1500 (b2f6553) into master (339c1d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1500   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files         863      863           
  Lines       41766    41779   +13     
=======================================
+ Hits        41024    41037   +13     
  Misses        742      742           

Impacted Files	Coverage Δ
lib/ddtrace/error.rb	97.87% <100.00%> (ø)
spec/ddtrace/error_spec.rb	94.06% <100.00%> (+0.73%)	⬆️

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[marcotc]

Thank you for this fix, @masato-hi! 🙇