cmd/fscrypt: fix missing protector error format

[hobbitalastair]

This is the bugette mentioned in #272 - the format specifiers were never being filled in.
Also add a newline so the prompt to select a protector shows on a separate line.

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[hobbitalastair]

@googlebot I signed it!

[ebiggers]

Could you add Update #272 to the commit message so that it directly links to the bug report?

[hobbitalastair]

@ebiggers thanks for the review, good point about the long line

I've called the formatted text loadWarning to be more descriptive, happy to change if you would rather the shorter version.

[ebiggers]

Merged, thanks!