templates: use inspect/1 instead of to_string/1 in errors_on/1

[v0idpwn]

Per elixir-ecto/ecto#4445

I don't see a reason to prefer to_string over inspect here, maybe I'm missing something :)

[viniciussbs]

So, I've tried inspect/1 in a custom errors_on/1 implementation in my code base. I gave up because decimal values get weird representation when interpolated using inspect/1:

iex(1)> to_string(Decimal.new(0))
"0"
iex(2)> inspect(Decimal.new(0))
"Decimal.new(\"0\")"

[v0idpwn]

Considering this function is built for testing, one could would argue that Decimal.new("0") is a more precise depiction of the data for the error message than 0.