Restructure parsing in augur export v2

[benjaminotter]

Description of proposed changes

Restructures the parsing of metadata and node data files in augur export_v2. The metadata and node data files are now loaded prior to calling the parser. The load calls are wrapped in two individual try/except blocks in order to print a specific error message if any of the files are missing.
The parser was modified to now take the already loaded files as input instead of loading the files itself.
Still need to check whether the parser function is referenced outside of the export module.

Related issue(s)

Fixes #733

Testing

no further doctests added
tested with the nextstrain Zika tutorial

[codecov]

Codecov Report

Merging #752 (967896a) into master (653079e) will decrease coverage by 0.02%.
The diff coverage is 8.33%.

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
- Coverage   31.52%   31.50%   -0.03%     
==========================================
  Files          41       41              
  Lines        5754     5758       +4     
  Branches     1389     1389              
==========================================
  Hits         1814     1814              
- Misses       3865     3869       +4     
  Partials       75       75              

Impacted Files	Coverage Δ
augur/export_v2.py	11.00% <8.33%> (-0.08%)	⬇️

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[huddlej]

This worked for me! Thank you, @benjaminotter!