Provide nicer parse errors and suggest "full-syntax"

[cuviper]

The included compile_fail test has this output:

error: expected `,`
  --> src/test.rs:13:24
   |
10 |     VarA = get_an_isize!(), // error without "syn/full"
   |                        ^

error: this might need the "full-syntax" feature of `num-derive`
 --> src/test.rs:11:10
  |
8 | #[derive(num_derive::FromPrimitive)]
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

Closes #16.

[cuviper]

bors r+

[bors]

Build succeeded:

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