rpcserver: Internal err on gettxout utxo fetch err.

[itswisdomagain]

Replace noTxInfoError with rpcInternalError when fetching utxo
entry errors in gettxout cmd handler. gettxout should never return
noTxInfoError because gettxout is about tx outputs not txs.

Update code comment and json-rpc docs to more clearly describe the
semantics of the gettxout method, particularly that JSON null is
returned if the requested txout cannot be found either because it
never existed or because it is spent (and possibly pruned).

[davecgh]

Please note that I changed the commit/PR title to match the code contribution guidelines.

[itswisdomagain]

Please note that I changed the commit/PR title to match the code contribution guidelines.

Thanks! Still figuring out the PR titling pattern.

[davecgh]

https://github.com/decred/dcrd/blob/master/docs/code_contribution_guidelines.md#ModelGitCommitMessages

Namely:

prefix: Short summary of changes (max 50 chars)

More detailed explanatory text, if necessary. Wrap it to about 72
characters or so. In some contexts, the first line is treated as the
subject of an email and the rest of the text as the body. The blank
line separating the summary from the body is critical (unless you omit
the body entirely); tools like rebase can get confused if you run the
two together.

...