Validate parameters

[gabrielpra1]

Add guard clauses to obtain better error messages.

For example, get_with_assocs passing the struct instead of ID gives

     ** (Ecto.Query.CastError) deps/ecto/lib/ecto/repo/queryable.ex:373: value `%Sppyns.Transactions.FundTransaction{__meta__: #Ecto.Schema.Metadata<:loaded, "fund_transactions">, amount: #Decimal<1>, fee: #Decimal<0.01>, fund: #Ecto.Association.NotLoaded<association :fund is not loaded>, fund_id: 3009, id: 1965, inserted_at: #DateTime<2019-06-28 18:10:26Z>, number_of_tokens: #Decimal<0.99>, operations: #Ecto.Association.NotLoaded<association :operations is not loaded>, transferred: nil, type: :invest, updated_at: #DateTime<2019-06-28 18:10:26Z>, user: #Ecto.Association.NotLoaded<association :user is not loaded>, user_id: 5457, valuations: #Ecto.Association.NotLoaded<association :valuations is not loaded>}` in `where` cannot be cast to type :id in query:

[gabrielpra1]

From the ecto docs:

@primary_key - configures the schema primary key. It expects a tuple {field_name, type, options} with the primary key field name, type (typically :id or :binary_id, but can be any type) and options. It also accepts false to disable the generation of a primary key field. Defaults to {:id, :id, autogenerate: true}.

So if anything can be a PK, I don't think it's possible to add any validation here