Merge lp://staging/~abentley/launchpad/bad-branch-name into lp://staging/launchpad

Treat bad branch location in recipe as field error

= Summary =
Fix bug #592792: bad branch names in recipes produce oopses

== Proposed fix ==
Catch NoSuchBranch when creating the SourcePackageRecipe and set a suitable
field error.

== Pre-implementation notes ==
None

== Implementation details ==
Extracted createRecipe so it could be reused.

Implemented bad branch handling for SourcePackageRecipeDataInstruction.

== Tests ==
bin/test -t test_create_recipe_bad_base_branch -t test_create_recipe_bad_instruction_branch

== Demo and Q/A ==
Attempt to create a recipe for a base branch that doesn't exist. This should
produce a validation error.

Attempt to create a recipe that merges a branchh that doesn't exist. This
should produce a validation error.

= Launchpad lint =

Checking for conflicts. and issues in doctests and templates.
Running jslint, xmllint, pyflakes, and pylint.
Using normal rules.

Linting changed files:
  lib/lp/code/model/sourcepackagerecipedata.py
  lib/lp/code/browser/sourcepackagerecipe.py
  lib/lp/code/browser/tests/test_sourcepackagerecipe.py

== Pyflakes notices ==

lib/lp/code/browser/tests/test_sourcepackagerecipe.py
    283: local variable 'recipe_fings' is assigned to but never used