Description of problem:
During build of packages, we are quite often using constructs such as:

```
%{?scl:scl enable %{scl} %{scl_nodejs} - << \EOF}
%gem_install -n %{SOURCE0}
%{?scl:EOF}
```

But if something goes wrong, the RPM build log contains just something like this:

```
+ scl enable rh-ror42 rh-nodejs4 -
/opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/defaults/operating_system.rb:131:in `default_dir': undefined method `[]' for nil:NilClass (NoMethodError)
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:38:in `default_specifications_dir'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:84:in `default_gem?'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/specification.rb:2079:in `base_dir'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:98:in `extensions_dir'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:91:in `extension_dir'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:243:in `require_paths'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:279:in `lib_dirs_glob'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/basic_specification.rb:269:in `matches_for_glob'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems.rb:468:in `block in find_latest_files'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems.rb:467:in `map'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems.rb:467:in `find_latest_files'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems.rb:995:in `load_plugins'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/gem_runner.rb:82:in `<top (required)>'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require'
  from /opt/rh/rh-ruby23/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require'
  from /opt/rh/rh-ruby23/root/usr/bin/gem:9:in `<main>'
error: Bad exit status from /var/tmp/rpm-tmp.7oUR6B (%prep)
```

It is not really clear what actually was executed and what could be wrong. It would be useful, if 'scl enable' had some verbose mode. Simple solution could be to prepend 'set -x' to script which is executed.