exceptions: set backtrace for FormulaUnreadableError

Library/Homebrew/exceptions.rb

@@ -208,6 +208,7 @@ class FormulaUnreadableError < FormulaUnavailableError
   def initialize(name, error)
     super(name)
     @formula_error = error
+    set_backtrace(error.backtrace)
   end
 end
 
@@ -257,6 +258,7 @@ class TapFormulaUnreadableError < TapFormulaUnavailableError
   def initialize(tap, name, error)
     super(tap, name)
     @formula_error = error
+    set_backtrace(error.backtrace)
   end
 end
 

Library/Homebrew/test/cli/named_args_spec.rb

@@ -4,7 +4,7 @@
 require "cli/named_args"
 
 def setup_unredable_formula(name)
-  error = FormulaUnreadableError.new(name, "testing")
+  error = FormulaUnreadableError.new(name, RuntimeError.new("testing"))
   allow(Formulary).to receive(:factory).with(name, force_bottle: false, flags: []).and_raise(error)
 end