improve the wording of the to_money deprecations warning

[elfassy]

Why

We want to track deprecations happening with the .to_money method seperatly from those from arithmetic computations. This will allow us to safely raise knowing these issues no longer happen in production

What

Introduce a new deprecation warning message for to_money with inconsistent currencies

Before:

Money.new(1, "USD").to_money("CAD") 
# "mathematical operation not permitted for Money objects with different currencies CAD and USD. A Money::IncompatibleCurrencyError will raise in the next major release"

After:

Money.new(1, "USD").to_money("CAD") 
# "to_money is attempting to change currency of an existing money object from USD to CAD. A Money::IncompatibleCurrencyError will raise in the next major release"

Note

This only affects those using the legacy_deprecations option. Others will continue to get an Money::IncompatibleCurrencyError exception

@Shopify/shopify-money

[lavoiesl]

Tagging in @chitty, who recently bumped Russbot to Money 2.0 and saw a lot of those warnings

[chitty]

Looks great, I also find the resulting code more readable 🙌