fix multiple upload and improve documentation (fix for issue-427)

[Maddogghoek]

Description

Fix for issue 427

• Allow multiple images to be uploaded in a single upload command (to match docs)
• improve error handling and reporting for invalid project ids
• Improve upload command documentation

List any dependencies that are required for this change.

Type of change

• [ x] Bug fix (non-breaking change which fixes an issue)
• [ x] New feature (non-breaking change which adds functionality)

How has this change been tested, please provide a testcase or example of how you tested the change?

manual testing,
roboflow upload --help [shows up to date documentation]
roboflow uplaod image.jpg [should fail with descriptive message]
roboflow -p project_id image.jpg [should succeed]
roboflow -p url/project_id image.jpg [should fail with descriptive message]
roboflow -p project_id image.jpg image2.jpg [should succeed]

Docs

New functionality matches what is listed in the docs)

[CLAassistant]

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❌ maddogghoek
❌ pre-commit-ci[bot]

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