Improve the error message when --embedded-database fails to start

[lostman]

When --embedded-database fails to start, we get a cryptic error message:

fuel-indexer run --embedded-database

Error: error_type: PgStartFailure
source:
None
message:
None

This may happen when the default postgres port is already in use by another service (e.g., system postgres).

Since this isn't obvious, we could either try to check that the port is already in use or print a more helpful error message indicating this may be the case.