fix(macho): Downgrade codesigning failures to a warning (#275)

This turned out to cause problems with certain exotic dylibs. We should allow
this to be caught by the consumer for recovery.

refs Homebrew/brew#8922

Author: mistydemeo

lib/macho.rb

@@ -56,6 +56,6 @@ def self.codesign!(filename)
            "--preserve-metadata=entitlements,requirements,flags,runtime",
            filename)
 
-    raise ModificationError, "#{filename}: signing failed!" unless $CHILD_STATUS.success?
+    raise CodeSigningError, "#{filename}: signing failed!" unless $CHILD_STATUS.success?
   end
 end

lib/macho/exceptions.rb

@@ -9,6 +9,11 @@ class MachOError < RuntimeError
   class ModificationError < MachOError
   end
 
+  # Raised when codesigning fails. Certain environments
+  # may want to rescue this to treat it as non-fatal.
+  class CodeSigningError < MachOError
+  end
+
   # Raised when a Mach-O file modification fails but can be recovered when
   # operating on multiple Mach-O slices of a fat binary in non-strict mode.
   class RecoverableModificationError < ModificationError