Capture Psych::SyntaxError and display a helpful message

[jcoyne]

$ solr_wrapper
/home/vagrant/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/psych.rb:377:in `parse': (<unknown>): could not find expected ':' while scanning a simple key at line 4 column 1 (Psych::SyntaxError)
    from /home/vagrant/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/psych.rb:377:in `parse_stream'
    from /home/vagrant/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/psych.rb:325:in `parse'
    from /home/vagrant/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/psych.rb:252:in `load'
    from /home/vagrant/.rvm/gems/ruby-2.3.1/gems/solr_wrapper-0.18.0/lib/solr_wrapper/configuration.rb:133:in `read_config'

Like "Your config file is invalid"