Crash parsing invalid file uri

[WetDesertRock]

Description:

storage/repository/parse.go assumed that there would always be a colon terminating the scheme. As seen in #3275 this led to an unexpected panic.

Note that I'm not as familiar with the usage of URI's in fyne, and this does make the code less lenient and throw errors more often (in theory). For example, "path/to/file" now expected to throw an error. After some research I think this is acceptable behavior as I don't see any way for those 'URI's to work. Let me know if this isn't a valid assumption and I can resubmit with a different fix in place (targeting just the reported issue)

Fixes #3275

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.

[coveralls]

Coverage decreased (-0.001%) to 61.41% when pulling 6d73ff8 on WetDesertRock:crash_parsing_invalid_file_URI into 23d1052 on fyne-io:develop.

[Jacalz]

Thanks for the PR. While the fix seems to work, I think the previous solution to getting the scheme was slow and unnecessary complex. It would be better if we just removed it in favour of a better solution. I have proposed one in the comments.

[Jacalz]

Thanks. Good call about the URI not necessarily having ://.

[Jacalz]

This looks good to me now. Thanks your for all the work that has gone into it and congratulations on your first contribution to Fyne 🙂