give a descriptive error for pipe tries with 'false'

If libapt has builtin support for a compression type it will create a
dummy compressor struct with the Binary set to 'false' as it will catch
these before using the generic pipe implementation which uses the
Binary. The catching happens based on configured Names through, so you
can actually force apt to use the external binaries even if it would
usually use the builtin support. That logic fails through if you don't
happen to have these external binaries installed as it will fallback to
calling 'false', which will end in confusing 'Write error's.
So, this is again something you only encounter in constructed testing.

Gbp-Dch: Ignore

apt-pkg/contrib/fileutl.cc

@@ -1882,6 +1882,9 @@ public:
 
       if ((Mode & FileFd::ReadWrite) == FileFd::ReadWrite)
 	 return filefd->FileFdError("ReadWrite mode is not supported for file %s", filefd->FileName.c_str());
+      if (compressor.Binary == "false")
+	 return filefd->FileFdError("libapt has inbuilt support for the %s compression,"
+	       " but was forced to ignore it in favor of an external binary – which isn't installed.", compressor.Name.c_str());
 
       bool const Comp = (Mode & FileFd::WriteOnly) == FileFd::WriteOnly;
       if (Comp == false)