EXP-78. Capture all errors when validating model
definitions and environments.

This commit captures and reports to the user all model
definition and environment validation errors.

Reviewed by: kiyan
EXP-93. Fixes bug in writing null values in the generic api.

Reviewed by: kiyan

issue: https://jira.kiji.org/EXP-93
Did code work, no tests yet.
KijiTapSuite working; KijiJobSuite left
KijiJobSuite tests, that don't error when expected yet.
added scratchwork
Two more tests working. Nonexistent instance still fails earlier.
more scratch work
tests pass.
slight refactoring