Improve README for troubleshooting "project does not exist"

[eeaton]

TL;DR

Improve troubleshooting guidance in the readme for errors related to a project ID stored in terraform state when the GCP project doesn't exist. This is particularly common when a previous run failed due to project quota issues.

Example of somebody getting stuck, and detailed description of the fix, at: #1187
#488 (comment)

Terraform Resources

n/a

Detailed design

n/a

Additional information

See also #451

Common scenarios where the build can fail during tf apply after passing terraform validate

• quota issues
• values for project labels contain illegal character (could also improve this by setting conditional validation at the tf variable to catch it sooner)
• terraform apply fails due to a timeout

[eeaton]

As I'm cleaning up the backlog of issues, I've found a few issues around this same idea. I'll close a few as duplicates and keep the single issue open to track work.

• [UX] Project ID collision handling #451
• [UX] Add guidance on recovering from apply time errors #488
• Creation of hierarchical firewall rules in 3-Network Hub & Spoke fails with "already exists" error #1190
• Stage 1 failing with "One or more users named in the policy do not belong to a permitted customer" #1257 ( not specifically about the project does not exist error, but similar as an edge case that happens when terraform fails midway through the apply state)