Make validation errors easier to fix

Problem: the warnings don't give enough details about which
values are problematic, only the name of the leaf key. This is
all the more annoying when you have a chart depending on other charts.

```
mainchart
 |
 +- subchart1
 +- subchart2
 +- subchart3
```

Here are some warnings I get before the change:
```
coalesce.go:199: warning: destination for credentials is a table. Ignoring non-table value
coalesce.go:160: warning: skipped value for resources: Not a table.
coalesce.go:160: warning: skipped value for googleSheetsServiceAccount: Not a table.
coalesce.go:199: warning: destination for googleSheetsServiceAccount is a table. Ignoring non-table value
coalesce.go:199: warning: destination for resources is a table. Ignoring non-table value []
coalesce.go:199: warning: destination for credentials is a table. Ignoring non-table value
coalesce.go:199: warning: destination for credentials is a table. Ignoring non-table value
coalesce.go:160: warning: skipped value for resources: Not a table.
coalesce.go:160: warning: skipped value for googleSheetsServiceAccount: Not a table.
```

with fix:
```
coalesce.go:162: warning: skipped value for subchart1.resources: Not a table.
coalesce.go:162: warning: skipped value for subchart2.googleSheetsServiceAccount: Not a table.
coalesce.go:211: warning: destination for subchart3.aws.credentials is a table. Ignoring non-table value ()
coalesce.go:211: warning: destination for mainchart.subchart3.aws.credentials is a table. Ignoring non-table value ()
coalesce.go:211: warning: destination for mainchart.subchart2.googleSheetsServiceAccount is a table. Ignoring non-table value ()
coalesce.go:211: warning: destination for mainchart.subchart1.resources is a table. Ignoring non-table value ([])
coalesce.go:162: warning: skipped value for subchart1.resources: Not a table.
coalesce.go:162: warning: skipped value for subchart2.googleSheetsServiceAccount: Not a table.
coalesce.go:211: warning: destination for subchart3.aws.credentials is a table. Ignoring non-table value ()
```

Signed-off-by: Damien Nozay <damiennozay+github@gmail.com>

add tests

Signed-off-by: Damien Nozay <damiennozay+github@gmail.com>
(cherry picked from commit 65ec3d6)