Add error messages for failing interpolations in project-factory

[wiktorn]

Currently, failed interpolations fail with the error message like this:

Error: invalid value for member (IAM members must have one of the values outlined here: https://cloud.google.com/billing/docs/reference/rest/v1/Policy#Binding)

  with module.projects-iam["service1"].google_project_iam_member.shared_vpc_host_iam["rw"],
  on ../project/shared-vpc.tf line 131, in resource "google_project_iam_member" "shared_vpc_host_iam":
 131:   member     = each.value

This is not helpful, when looking for a source of the issue. With this PR, error messages look like this:

Error: Error in function call

  on main.tf line 173, in module "projects-iam":
 173:         try(
 174:           # project service accounts
 175:           module.service-accounts["${each.key}/${v}"].iam_email,
 176:           # automation service account
 177:           local.context.iam_principals["${each.key}/${v}"],
 178:           # other projects service accounts
 179:           module.service-accounts[v].iam_email,
 180:           # other automation service account
 181:           local.context.iam_principals[v],
 182:           # passthrough + error handling using tonumber until Terraform gets fail/raise function
 183:           strcontains(v, ":") ? v : tonumber("[Error] Invalid member: '${v}' in project '${each.key}'")
 184:         )
    ├────────────────
    │ each.key is "service1"
    │ local.context.iam_principals is object with 3 attributes
    │ module.service-accounts is object with 4 attributes

Call to function "try" failed: no expression succeeded:
- Invalid index (at main.tf:175,34-54)
  The given key does not identify an element in this collection value.
- Invalid index (at main.tf:177,39-59)
  The given key does not identify an element in this collection value.
- Invalid index (at main.tf:179,34-37)
  The given key does not identify an element in this collection value.
- Invalid index (at main.tf:181,39-42)
  The given key does not identify an element in this collection value.
- Invalid function argument (at main.tf:183,47-72)
  Invalid value for "v" parameter: cannot convert "[Error] Invalid member: 'rw' in project 'service1'" to number; given string must be a decimal representation of a number.

At least one expression must produce a successful result.

Which provides the user with the information that allows them to narrow down the search area.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass