Always ensure valid incremental strategy

[QMalcolm]

resolves #330

Problem

Previously if you set an invalid incremental_strategy for an incremental model, it'll succeed on the first run, and then fail on the second run
I.e. if you have

{{config(materialized="incremental", incremental_strategy="bad_strategy")}}

select * from {{ ref("input_model") }}

{% if is_incremental() %}
where event_time >= (select coalesce(max(event_time),'1900-01-01') from {{ this }} )
{% endif %}

That was always succeeding when the model was either built for the first time or rebuilt via --full-refresh. You only got an error about the strategy the next time the model was run.

Solution

Change the order of operations in incremental.sql to ensure that a incremental model's incremental_strategy is always validated

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX