[MRG] autodetect FASTA/FASTQ files if given as signatures

[ctb]

Raise a nicer exception if the file given to _load_databases can be loaded as a FASTA/FASTQ file.

Fixes #460

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1078 into master will increase coverage by 0.84%.
The diff coverage is 93.85%.

@@            Coverage Diff             @@
##           master    #1078      +/-   ##
==========================================
+ Coverage   92.06%   92.91%   +0.84%     
==========================================
  Files          72       74       +2     
  Lines        5397     5702     +305     
==========================================
+ Hits         4969     5298     +329     
+ Misses        428      404      -24     

Impacted Files	Coverage Δ
sourmash/signature.py	92.66% <66.66%> (+1.91%)	⬆️
sourmash/lca/command_classify.py	85.71% <75.00%> (+0.38%)	⬆️
sourmash/lca/lca_utils.py	92.80% <85.00%> (-3.76%)	⬇️
sourmash/commands.py	86.64% <91.66%> (+0.38%)	⬆️
sourmash/lca/command_summarize.py	90.00% <91.83%> (+2.78%)	⬆️
sourmash/sig/__main__.py	93.38% <93.50%> (+0.67%)	⬆️
sourmash/sourmash_args.py	95.37% <94.96%> (+0.43%)	⬆️
sourmash/__init__.py	100.00% <100.00%> (ø)
sourmash/cli/compare.py	100.00% <100.00%> (ø)
sourmash/cli/gather.py	100.00% <100.00%> (ø)
... and 29 more

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[bluegenes]

again, this review contains a "hey @ctb, why did you you do a thing a certain way??" :)

[bluegenes]

why is it better to pass here, then try again below (rather than try/except within the exception)?

[ctb]

in this case, because we have so many things to try. if we did nested try/excepts it would end up indenting to the right.

[bluegenes]

great, thanks!