Misleading error message in Flatten

[tsnowlan]

Related to: #82

Config and files used are arbitrary, just make a typo in the path for file in config toml and -base-path.

When attempting to open a Source file, the basepath is prepended twice. The first time in Sources, the next time in Flatten. This causes the error string to be [Flatten] unable to open file: basepath/basepath/filename in basepath. which implies that the basepath string was doubled somewhere rather than basepath/filename also not existing.

PR incoming with proposed fix.

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If you have encountered an error, please include:

• [x] minimal conf and lua files that you are using.
• [x] urls or actual files for annotations in conf file.
• [x] minimal query file.
• [x] the command you used to invoke vcfanno
• [x] the full error message

[tsnowlan]

For the sake of completeness, from example/conf.toml: using bad_conf.txt (github doesn't like .toml extension)

~/vcfanno$ vcfanno -base-path example -lua custom.lua example/bad_conf.txt example/query.vcf.gz

=============================================
vcfanno version 0.3.2 [built with go1.13.1]

see: https://github.com/brentp/vcfanno
=============================================

ERROR: can't find query file: example/example/query.vcf.gz

[brentp]

closed in #119