Give a more user-friendly diagnostic about the following:
* Trailing tokens within braces, e.g. `${foo() extra}`
* Missing parentheses, e.g. `${foo}`
* Incorrect number of arguments, with a hint about correct usage.
`syntax-errors` currently contains both syntax error tests (which don't
need expansion) and tests for incorrect instantiations of the `count`
metavariable expression (which do need expansion). Split the
expansion-dependent tests to a separate file and remove unneeded
invocations from `syntax-errors`, to ensure we are catching these before
expansion.
These tests have expanded beyond the RFC, so rename the directory
`rfc-3086-metavar-expr` to `metavar-expressions`. `concat` (which wasn't
part of the RFC) now fits in this group, so merge its tests into the
`metavar-expressions` directory.
Additionally rename some related `issue-*` tests.
