navajo: Brings `envelope::sync::Envelope` in line with `Envelope`

[chanced]

currently, the async `Envelope trait's associated error types are:

pub trait Envelope {
    type EncryptError: core::fmt::Display + core::fmt::Debug + Send + Sync;
    type DecryptError: core::fmt::Display + core::fmt::Debug + Send + Sync;
    // ...
}

while the sync version is:

pub trait Envelope {
    type EncryptError: super::Error + Send + Sync;
    type DecryptError: super::Error + Send + Sync;
    // ...
}

This brnigs both in line by replacing super::Error with Display + Debug

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.03 :warning:

Comparison is base (456cd2e) 73.34% compared to head (cf8bc54) 73.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================== - Coverage 73.34% 73.31% -0.03% ========================================== Files 93 93 Lines 10916 10916 ========================================== - Hits 8006 8003 -3 - Misses 2910 2913 +3 ``` | [Impacted Files](https://codecov.io/gh/chanced/navajo/pull/71?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance) | Coverage Δ | | |---|---|---| | [navajo/src/envelope.rs](https://codecov.io/gh/chanced/navajo/pull/71?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance#diff-bmF2YWpvL3NyYy9lbnZlbG9wZS5ycw==) | `75.16% <ø> (ø)` | | ... and [2 files with indirect coverage changes](https://codecov.io/gh/chanced/navajo/pull/71/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.