Raise error on underspecified path for import/export and view

[cbkerr]

Description

Just the bug fix.

There is some duplicate code, but that will be refactored in the rest of #642.

Motivation and Context

see #642 for a description

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #666 (6e29705) into master (8176ba9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 6e29705 differs from pull request most recent head fa86a4e. Consider uploading reports for the commit fa86a4e to get more accurate results

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   78.40%   78.42%   +0.02%     
==========================================
  Files          65       65              
  Lines        7112     7120       +8     
  Branches     1561     1564       +3     
==========================================
+ Hits         5576     5584       +8     
  Misses       1226     1226              
  Partials      310      310              

Impacted Files	Coverage Δ
signac/contrib/linked_view.py	83.91% <ø> (ø)
signac/contrib/import_export.py	78.36% <100.00%> (+0.46%)	⬆️

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[bdice]

I have a couple small suggestions. Otherwise this looks good.

[bdice]

Thanks @cbkerr. I pushed some changes and will set this to automerge once CI passes.