localrepo: fix conversion of exceptions to strings flagged by pytype
This is the same as e571fec5b606. I assume the 3 similar uses of `bytestr` in
`__getitem__()` are OK (even though pytype also flags them), because there is
apparently test coverage for 2 of the 3 cases.
Differential Revision: https://phab.mercurial-scm.org/D10131
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}