Try import statements until one succeeds.

    Parameters
    ----------
    imports : list
        The list of import statements.
    fail_msg : str
        If not None and no statement succeeds, a `ValueError` is raised with
        the given message, appended to all failed messages.

    Returns
    -------
    locals : dict
        The dictionary of imported modules.

def try_imports(imports, fail_msg=None):
    """
    Try import statements until one succeeds.

    Parameters
    ----------
    imports : list
        The list of import statements.
    fail_msg : str
        If not None and no statement succeeds, a `ValueError` is raised with
        the given message, appended to all failed messages.

    Returns
    -------
    locals : dict
        The dictionary of imported modules.
    """
    msgs = []
    for imp in imports:
        try:
            exec imp
            break

        except Exception, inst:
            msgs.append(str(inst))

    else:
        if fail_msg is not None:
            msgs.append(fail_msg)
            raise ValueError('\n'.join(msgs))

    return locals()