Fix spaces in Mothur taxonomy strings

This fixes a problem that we discovered while assigning taxonomy with Mothur on the QIIME 1.7 VM.

Problem
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Assigning taxonomy with Mothur on the QIIME 1.7 VM fails with a KeyError.  A couple forum posts and at least one bug (closed) are related to this issue:

  - https://groups.google.com/d/topic/qiime-forum/BFzZTXT2r4M/discussion
  - https://groups.google.com/d/topic/qiime-forum/-eyC4Fwzt0w/discussion
  - https://github.com/qiime/qiime/issues/1002
  - https://github.com/qiime/qiime/issues/702

Output
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The traceback from QIIME and Mothur log file were similar to that described by Leah Reshef in the first forum post above.

Solution
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Mothur does not allow any spaces in the taxonomy string (see http://www.mothur.org/wiki/Taxonomy_outline).  We now replace spaces with underscores and remove spaces surrounding the ";" taxon separators before giving the taxonomy string to Mothur.

Additionally, we avoid the KeyError if results are not produced by Mothur.  Instead, we return a RuntimeError reporting that no output file was produced by Mothur.

Workaround
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This problem can also be solved by modifying the GreenGenes taxonomy files with a perl script.  On the QIIME 1.7 VM, a user can run:

    perl -pe's/; /;/g;s/ /_/g' ~/qiime_software/gg_otus-12_10-release/taxonomy/97_otu_taxonomy.txt > ~/qiime_software/gg_otus-12_10-release/taxonomy/97_otu_taxonomy_nospaces.txt

Then, the `97_otu_taxonomy_nospaces.txt` file can be used via the `--id_to_taxonomy_fp` option when running `assign_taxonomy.py`.