nodejs plugin: build fails if there are missing peer dependencies.

[TheBITLINK aka BIT]

This happens when trying to run `npm ls --json` at the build stage.

If there are any missing peer dependencies, the command exits with a non-zero exit code.

However, most of the time those errors can be safely ignored.

An attempt to mitigate this is already in place:

```
def _get_installed_node_packages(self, cwd):
    # There is no yarn ls
    cmd = [os.path.join(self._npm_dir, "bin", "npm"), "ls", "--json"]
    try:
        output = self.run_output(cmd, cwd)
    except subprocess.CalledProcessError as error:
        # XXX When dependencies have missing dependencies, an error like
        # this is printed to stderr:
        # npm ERR! peer dep missing: glob@*, required by glob-promise@3.1.0
        # retcode is not 0, which raises an exception.
        output = error.output.decode(sys.getfilesystemencoding()).strip()
```

However, `run_output()` itself captures the `CalledProcessError`, rasing a different type of exception (SnapcraftPluginCommandError):

```
def run_output(self, cmd, cwd=None, **kwargs):
    if not cwd:
        cwd = self.builddir
    os.makedirs(cwd, exist_ok=True)
    try:
        return common.run_output(cmd, cwd=cwd, **kwargs)
    except CalledProcessError as process_error:
        raise errors.SnapcraftPluginCommandError(
            command=cmd, part_name=self.name, exit_code=process_error.returncode
        ) from process_error
```