Change approach for detecting invalid targets

Initially (7374840), snakemake-check-target only used regular
expressions to detect invalid targets based on the presence of a
MissingRuleException or RuleException in Snakemake's output.  The target
was assumed to be valid if these exceptions weren't found.  If there was
a non-zero exit status for another reason, it bubbled up to the compile
call where it was visible to the user.

33a7c90 (snakemake-check-target: Adjust for upstream output, 2016-09-01)
restricted the invalid target check to calls with an exit status of
zero.  This makes the regular expression check useless because snakemake
should always exit with a non-zero status if a MissingRuleException or
RuleException is thrown.  Due to this change, snakemake-check-target
classified all non-zero exits as invalid and all zero exits as valid.
While this often gives the right answer, it doesn't in cases where the
non-zero exit is unrelated to an invalid target.
2bceb7f (snakemake-check-target: Recognize protected items, 2016-09-05)
addressed one case.

To deal with other cases (such as an ambiguous rule error or a syntax
error in the Snakefile), use the following approach.

* An exit status of zero indicates a valid target.

* A non-zero exit status indicates an invalid target if
  snakemake-all-rules has an exit status of zero.  Otherwise,
  snakemake-all-rules will signal an error and display the Snakemake
  output.

The main downside of this approach is the need to call snakemake twice.
The output of snakmake-all-rules is cached, so this is only the case on
the first call to snakemake-check-target for a given version of a
Snakefile.

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