Remove Error::Generic variants in favor of more specific errors

[insipx]

Generic should not be used, instead we should prefer creating a new enum variant in almost all cases. Using "Generic" errors may lead to depending on strings in an a variant leading to difficult to find bugs.

If a string changes in one part of a codebase, it could inadvertently lead to very bad things happening if the author is not careful (silenced errors no longer becoming silenced, retryable errors no longer being retried, etc.).

Enum variants will give a compile time error and force the author to fix.

re: this comment:

libxmtp/xmtp_mls/src/storage/errors.rs

Line 60 in cd0fc5c

// TODO: Figure out the full list of non-retryable errors.

[neekolas]

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