Document causes of common errors

We should document common errors users get when executing queries. The following are the top 11:

1. undefined identifier,249281
1. failed to initialize execute state: could not find bucket,103195
2. compilation failed: error invalid expression $,16560
3. cannot access element 0 of array of length 0,10370
4. failed to initialize execute state: bucket not found,8008
5. compilation failed: error regex parse error: \b${Boat:regex}\b error: repetition quantifier expects a valid decimal,7338
6. An internal error has occurred,6592
7. "error calling function ""get"" secret not found",6070
8. "error in query specification while starting program: this Flux script returns no streaming data. Consider adding a ""yield"" or
9. invoking streaming functions directly, without performing an assignment",5884
10. too many requests (): exceeded rate limit; too many requests (): exceeded rate limit,5108

For the full top 50, see: https://docs.google.com/document/d/1ZkCxLQpZwZdqB1vyh5Uo2trOr6UCAPooHKn2EDF5iwA/edit

No documentation file or test is named. Start by reviewing the listed errors and the linked top-50 source, then locate the relevant error-documentation area; done means the requested common errors have documented causes.

Written by the indexing model from the issue text.