lang: provide more helpful error when system_program is missing for init

[paul-schaaf]

currently:

Compiling token-omnibus v0.1.0 (/Users/aeyakovenko/token-omnibus/programs/token-omnibus)
error[E0425]: cannot find value `system_program` in this scope
  --> programs/token-omnibus/src/lib.rs:20:10
   |
20 | #[derive(Accounts)]
   |          ^^^^^^^^ not found in this scope

should be:

the init constraint requires the system_program to exist in the account validation struct. add the following to your struct: pub system_program: Program <'info, System>