Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
When route are given as Array, Angular throws error with Invalid route configuration: routes must have path specified which does not really express what's wrong.

What is the new behavior?
Throw error with new message Invalid route configuration: Array cannot be specified.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:
I'm certain that there are some people who think route can be given as array and it should be eventually flattened.
I hope this will them to figure out the problem they are having.
(Current error message confused me a little bit, and took sometime to resolve it.)