test_raise_error fails with musl libc

[yu-re-ka]

The test makes the assumption that getting the errno text for -1 will return Unknown error -1, but this is not defined anywhere in POSIX or C standards and other libc implementations have different return values for this. musl libc returns No error information.
I would suggest to remove the test variant or loosen the expectation of a specific error text.

[kurtmckee]

@yu-re-ka, please test the fix in #923 when you have an opportunity. Thanks!