`.IsError(...)` predicate could be more flexible

[maargenton]

Currently, .IsError(...) only accepts either nil to check there is no error, or another error that must match directly or indirectly through error wrapping.

Suggested extended behavior:

.IsError(nil)       // no error -- currently supported
.IsError()          // any error
.IsError(sentinel)  // match specific error -- currently supported
.IsError(str)       // error message contains string
.IsError(regex)     // error message matches regexp

Redefining it a IsError(match ...any) preserves backward compatibility (currently accepting 1 error argument), supports the new 0 argument case, and supports non-error expectations. Specifying more than 1 argument or unsupported argument types can be handled as an error. Similar pattern is used in other frameworks (e.g. fiber) to simulate optional argument.

[maargenton]

Resolved in v1.4.0