@Kha we are getting better error message, but we need to figure out a
way to suppress error messages that come from the "plumbing".
For example, in the following example.
```
def f4 (b : Bool) (n : Nat) (v : Vector Nat n) : Vector Nat (n+1) := do
if b then
v := Vector.cons 1 v
Vector.cons 1 v
```
We get the nice "invalid reassignment" error at `v := ...`, but
we also get the nasty
```
error: application type mismatch
jp✝ v
argument
v
has type
Vector Nat (n + 1)
but is expected to have type
Vector Nat n
failed to synthesize instance
CoeT (Vector Nat (n + 1)) v (Vector Nat n)
```
Where `jp✝` is a joinpoint created by the do-notation macro expander.
I thought about using `withoutErrToSorry` when elaborating the expanded
`do` term.
We may also try to add more helper hints such as `ensureTypeOf!` and
use them around "plumbing" code (e.g., "joinpoint goto"s)
