feat(Secretbox): throw fallback error when no encryption package installed

[amishshah]

Please describe the changes this PR makes and why it should be merged:

Resolves #128.

If no valid encryption package is installed, the following error is now thrown (rather than the obscure error shown in #128):

    throw new Error(`Cannot play audio as no valid encryption package is installed.
    ^

Error: Cannot play audio as no valid encryption package is installed.
- Install sodium, libsodium-wrappers, or tweetnacl.
- Use the generateDependencyReport() function for more information.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes

[codecov[bot]]

Codecov Report

Merging #130 (07efcd3) into main (e9c3a94) will increase coverage by 0.41%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   66.25%   66.66%   +0.41%     
==========================================
  Files          19       19              
  Lines         880      882       +2     
  Branches      214      214              
==========================================
+ Hits          583      588       +5     
+ Misses        295      292       -3     
  Partials        2        2              

Impacted Files	Coverage Δ
src/util/Secretbox.ts	57.89% <66.66%> (+22.60%)	:arrow_up:

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