chore: improve error message for invalid ipfs paths

[shushen]

• Fix Parsing "/" results in 'Path does not start with "/"' #35 and related to Wrong error: invalid path "/testfs": path does not begin with '/' kubo#8724
• Fix /<cid>/path/to/file paths are not considered valid paths #19

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[aschmahmann]

Looks great, thanks 🙏