Support for Darwin Core Archives without metafile

[niconoe]

According to the standard, you can have a perfectly valid DwC-A without a meta.xml given that:

• there's only a (core) data file
• that data file has one header row
• the Metadata is kept in a file named EML.xml

I tried to convert a test archive, but I got:

Traceback (most recent call last):
  File "/anaconda/envs/fdwca_pip_test/bin/fdwca", line 10, in <module>
    sys.exit(cli())
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/FrictionlessDarwinCore/fdwca.py", line 13, in cli
    da.save(outpath)
  File "/anaconda/envs/fdwca_pip_test/lib/python3.7/site-packages/FrictionlessDarwinCore/archive.py", line 74, in save
    zf.writestr('readme.md', self.metadata.as_markdown())
AttributeError: 'NoneType' object has no attribute 'as_markdown'

[andrejjh]

In a first step, I suggest to have a more explicit error saying 'no meta.xml found'.