message: Assert not only an error happens but the right error happens
  Add message assertions to the various assertions in `test.rs`
diff: +use failure::Error;
  use std::env;
  use utils::fixture;
  -use wasm_pack::command::{self, build, test, Command};
  -use wasm_pack::logger;
  +use wasm_pack::command::{build, test, Command};
  +
  +fn assert_err<T>(result: Result<T, Error>, msg: &str) -> Error {
  + let error = result.err().expect("should have failed");
  + for e in error.iter_chain() {
  + println!("err: {}", e);
  + }
  + assert!(error.iter_chain().any(|e| e.to_string().contains(msg)));
  + error
  +}
  #[test]
  fn it_can_run_node_tests() {
  @@ -89,11 +98,7 @@ fn it_can_run_failing_tests() {
  mode: build::BuildMode::Noinstall,
  ..Default::default()
  });
  - let logger = logger::new(&cmd, 3).unwrap();
  - assert!(
  - command::run_wasm_pack(cmd, &logger).is_err(),
  - "failing tests should return Err"
  - );
  + assert_err(fixture.run(cmd), "Running Wasm tests with wasm-bindgen-test failed");
  }
  #[test]
  @@ -147,11 +152,7 @@ fn it_requires_node_or_a_browser() {
  // Note: not setting node or any browser to true here.
  ..Default::default()
  });
  - let logger = logger::new(&cmd, 3).unwrap();
  - assert!(
  - command::run_wasm_pack(cmd, &logger).is_err(),
  - "need to enable node or browser testing"
  - );
  + assert_err(fixture.run(cmd), "Must specify at least one of");
  }
  #[test]
  @@ -166,11 +167,7 @@ fn the_headless_flag_requires_a_browser() {
  headless: true,
  ..Default::default()
  });
  - let logger = logger::new(&cmd, 3).unwrap();
  - assert!(
  - command::run_wasm_pack(cmd, &logger).is_err(),
  - "running headless tests in node doesn't make sense"
  - );
  + assert_err(fixture.run(cmd), "only applies to browser tests");
  }
  #[test]
  @@ -208,13 +205,7 @@ fn complains_about_missing_wasm_bindgen_test_dependency() {
  mode: build::BuildMode::Noinstall,
  ..Default::default()
  });
  - let logger = logger::new(&cmd, 3).unwrap();
  -
  - let result = command::run_wasm_pack(cmd, &logger);
  - assert!(
  - result.is_err(),
  - "running tests without wasm-bindgen-test won't work"
  - );
  + let error = assert_err(fixture.run(cmd), "Ensure that you have");
  // Test that the error message has two occurrences of "wasm-bindgen-test" in
  // it. I am surprised to learn there is no `str` method to count
  @@ -239,7 +230,7 @@ fn complains_about_missing_wasm_bindgen_test_dependency() {
  // the output to a text file, then the escape codes go away, so I can't
  // figure out which exact escape codes are even used here.
  - let err_msg = result.unwrap_err().to_string();
  + let err_msg = error.to_string();
  let first = err_msg.find("wasm-bindgen-test");
  assert!(first.is_some());
  let second = err_msg.rfind("wasm-bindgen-test");
  