Provide better guidance on conda create error

This comes out of our discussion on Flowdock and PR #963.  This provides
the user with an obvious next step to perform.
ENH: updated plan to use an instruction mapping instead of an if statement
TEST: added unittests for instructions refactor
fix more tests
flake8ification
Extract InvalidInstruction into exceptions and test it
Fix name typo
Say goodbye to .message :-(
FIX: plan_from_actions
TEST: updated tests to test plan_from_actions
FIX: tests
REF: revert changes for a later commit
REF: revert changes for a later commit
Merge branch 'feature/instructions' of https://github.com/conda/conda into feature/instructions
FIX: progress.stop now gets called
FIX: changed all plan.INSTRUCTION to inst.INSTRUCTIONS
Add this back in for BC
TEST: added tests for progress bar
Update CHANGELOG
Merge remote-tracking branch 'conda/master' into feature/instructions