Cannot execute workflow with no genome build set

I'm attempting to run an old workflow that refers to a bed file which does not have a genome build set (listed as ?)
The workflow fails in an unfriendly way (something went wrong).
Setting the genome build on the bed file does allow the workflow to run.

Could be reproduced with a very simpel workflow containing only a bedtools cov.
If we are now validating that all inputs have a genome build set, I think we should have a friendly error message. But I don't think it's a great idea to fail in the event of a missing genome build.

backtrace below

```
Traceback (most recent call last):
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/tools/__init__.py", line 1731, in check_and_update_param_values_helper
input.validate( value, history=trans.history )
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/tools/parameters/basic.py", line 2137, in validate
do_validate( value )
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/tools/parameters/basic.py", line 2121, in do_validate
validator.validate( v, history )
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/tools/parameters/validation.py", line 284, in validate
raise ValueError( self.message )
ValueError: Unspecified genome build, click the pencil icon in the history item to set the genome build
galaxy.tools.execute WARNING 2016-02-29 00:36:14,369 There was a failure executing a job for tool [toolshed.g2.bx.psu.edu/repos/aaronquinlan/bedtools/bedtools_coveragebe
d_counts/0.1.0] - {'inputB': "Value no longer valid for 'overlap the intervals in this BED file (target)', replacing with default"}
galaxy.tools.execute DEBUG 2016-02-29 00:36:14,369 Executed all jobs for tool request: (91.331 ms)
galaxy.workflow.run ERROR 2016-02-29 00:36:14,374 Failed to schedule Workflow[id=2700,name=Custom Genome DNA PE + Agg], problem occurred on WorkflowStep[index=4,type=too
l].
Traceback (most recent call last):
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/workflow/run.py", line 162, in invoke
jobs = self._invoke_step( step )
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/workflow/run.py", line 228, in _invoke_step
jobs = step.module.execute( self.trans, self.progress, self.workflow_invocation, step )
File "/mnt/galaxy/data/galaxy/galaxy-dist/lib/galaxy/workflow/modules.py", line 1188, in execute
raise Exception(message)
Exception: Failed to create 1 out of 1 job(s) for workflow step.
galaxy.workflow.run ERROR 2016-02-29 00:36:14,394 Failed to execute scheduled workflow.
```

Start by reproducing the minimal workflow containing only bedtools cov with a BED history item whose genome build is ?. Read galaxy/tools/parameters/basic.py and validation.py around the reported validation failure, then trace scheduling in galaxy/workflow/run.py. Confirm the expected behavior with maintainers, since the issue proposes either allowing the workflow to run or returning a friendlier error.

Written by the indexing model from the issue text.