replace panics with explicit ereport and error code

[conradludgate]

panic! is already being turned into error level reports, but with a generic error code. This makes it explicit.

[tristan957]

I appreciate this PR. Thanks!

[mrl5]

so to leave an approval with a clear conscience I'll just mention few refs that back up this implementation (also having in mind concern about "ereport and panic" in #6):

• ereport! for ERROR level will eventually panic anyway so these changes don't alter the existing runtime behavior
  • code ref1 https://docs.rs/pgrx-pg-sys/0.11.3/src/pgrx_pg_sys/submodules/elog.rs.html#342
  • code ref2 https://docs.rs/pgrx-pg-sys/0.11.3/src/pgrx_pg_sys/submodules/panic.rs.html#293-299
  • code ref3 https://docs.rs/pgrx-pg-sys/0.11.3/src/pgrx_pg_sys/submodules/panic.rs.html#157-158
• calling rust's panic (directly or indirectly) seems to be a proper way for pgrx, as per https://github.com/eeeebbbbrrrr/pgrx/blob/master/DESIGN-DECISIONS.md#protecting-the-rust-stack-read-lying-to-postgres
  • esp. this fragment:

    After Rust's stack has unwound, the panic!() is handed off to ereport(), passing final error handling control back to Postgres.