Print all schema violations in `validate`'s error message

[hannahilea]

Close #56.

# Setup
using Legolas
using Legolas: SchemaVersion, @schema, @version
using Tables

@schema "foo" Foo
@version FooV1 begin
    a::Vector
    b::AbstractString
    c::Int32
    d::Int32
end
f = FooV1SchemaVersion()
t = Tables.Schema((:a, :b), (Int32, Float64))
Legolas.validate(t, f)

main:

julia> Legolas.validate(t, f)
ERROR: ArgumentError: field `a` has unexpected type; expected <:Vector, found Int32
Stacktrace:
 [1] validate(ts::Tables.Schema{(:a, :b), Tuple{Int32, Float64}}, sv::SchemaVersion{:foo, 1})
   @ Legolas ~/Code/Legolas.jl/src/schemas.jl:285
 [2] top-level scope
   @ REPL[8]:1

This branch:

julia> Legolas.validate(t, f)
ERROR: ArgumentError: Tables.Schema violates Legolas schema `foo@1`:
 - Could not find required field: `c`
 - Could not find required field: `d`
 - Incorrect type: `a` expected `<:Vector`, found `Int32`
 - Incorrect type: `b` expected `<:AbstractString`, found `Float64`
Provided Tables.Schema:
 :a  Int32
 :b  Float64

(Note: was updated following review feedback below)

---

Punch list:

• [x] capture for multiple violations
• [x] show multiple violations in validation script
  • [x] example old version
  • [x] example new version
• [x] add/update tests
• [x] bump version