Better help answers for strange inputs (#33444)
Before:
```julia
help?> ``
ERROR: LoadError: Invalid at-var syntax `Core.at-cmd`.
help?> {a
ERROR: LoadError: Invalid at-var syntax `incomplete: premature end of input`.
help?> 123456789012345678901234567890
ERROR: LoadError: Invalid at-var syntax `Core.at-int128_str`.
```
After:
```julia
help?> ``
at-cmd str
...
help?> {a
No documentation found.
help?> 123456789012345678901234567890
at-int128_str str
...
```
