Introduce spi_xxx_or_error wrappers

I have seen that the pattern we follow to use `spi_execute` & `spi_finish` seems a lot similar in some places.

```c
if (spiResult != SPI_OK_UTILITY)
{
ereport(ERROR, (errmsg("could not run SPI query")));
}

if (spiResult != SPI_OK_FINISH)
{
ereport(ERROR, (errmsg("could not finish SPI connection")));
}
```

Might be better if we can introduce wrappers around spi_execute & spi_finish to automatically error out without doing those extra checks.