chore(cli): suppress non-blocking RPC errors in Semgrep logs at defau… (semgrep/semgrep-proprietary#6422)

…lt log level by logging at verbose

There are some recurring RPC log error messages that come from
validating rules that do not affect scan execution or results. Changed
logging level from error to verbose for the error lines that I could
find in the codebase. The remaining ones are either already at the
verbose/debug level or couldn't be found.

Test plan:
Temporarily changed `validate` function in `RPC_return.ml` to throw an
error, then ran `semgrep scan —config p/default .` with and without the
`--verbose` flag to ensure that the validate function ran. Checked that
the relevant lines only appear under verbose.

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