Add exit_status to cli error to fix #291

[konstin]

fixes #291

Example output:

  [1/9] Checking `rustc` version...
  [2/9] Checking crate configuration...
  [3/9] Adding WASM target...
| [4/9] Compiling to WASM...
Process exited with exit code: 101: Compilation of your program failed. stderr:

   Compiling a v0.1.0 (/home/konsti/wasm-pack/a)
error[E0425]: cannot find function `fail` in this scope
 --> src/lib.rs:3:5
  |
3 |     fail();
  |     ^^^^ not found in this scope

error: aborting due to previous error

For more information about this error, try `rustc --explain E0425`.
error: Could not compile `a`.

  [1/9] Checking `rustc` version...
  [2/9] Checking crate configuration...
  [3/9] Adding WASM target...
/ [4/9] Compiling to WASM...
Process exited with signal: 9: Compilation of your program failed. stderr:

   Compiling lazy_static v1.1.0
   Compiling log v0.4.5
   Compiling wasm-bindgen v0.2.23
   Compiling proc-macro2 v0.4.19
   Compiling ryu v0.2.6
   Compiling serde v1.0.79

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed and have your
  cloned directory set to nightly
• You ran rustfmt on the code base before submitting
• You reference which issue is being closed in the PR text

[fitzgen]

Looks great! THanks!