Add basic error handling for things like "couldn't find xcodeproject", "missing target in project", etc.

[jonnolen]

This is an example of an unhelpful error. Where I forgot to edit the name of the target. This is where ideally it would say "MyProj doesn't exist as a target in <my.xcodeproj>. It does contain the following targets: TargetA, TargetB..."

$ bundle exec integratej2objc integrate_source -p Xcode -x TapTap.xcodeproj -s Xcode/j2objc-generated/ -g generated -t MyProj
/Users/brunobowden/.bundler/ruby/2.0.0/integratej2objc-efcf2d333ce1/bin/integratej2objc:40:in integrate_source': undefined methodname' for nil:NilClass (NoMethodError)
from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/command.rb:27:in run' from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:ininvoke_command'
from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor.rb:359:in dispatch' from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/base.rb:440:instart'
from /Users/brunobowden/.bundler/ruby/2.0.0/integratej2objc-efcf2d333ce1/bin/integratej2objc:120:in <top (required)>' from /Library/Ruby/Gems/2.0.0/bin/integratej2objc:23:inload'
from /Library/Ruby/Gems/2.0.0/bin/integratej2objc:23:in `

'