fix(cli): fail on formatting errors during text output

[wackywendell]

Return a terminal error when text formatting reports errors in convert/validate while keeping best-effort output.

Also a bit of code cleanup.

Before

❯ cargo run --features=cli convert -i ~/data/where-42.substrait.json -t text
   Compiling substrait-explain v0.2.0 (/Users/wendell.smith/go/src/github.com/DataDog/substrait-explain)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.56s
     Running `target/debug/substrait-explain convert -i /Users/wendell.smith/data/where-42.substrait.json -t text`
=== Plan
Root["?column?"]
  Project[42]
    Filter[!{RexType} => ]
      Read[!{ReadRel} => ]
[ Process Exit Code: 0 ]

Note that !{RexType} and !{ReadRel} are not recognized, but no warnings are printed; also exit code 0 is for 'success'.

After

❯ cargo run --features=cli convert -i ~/data/where-42.substrait.json -t text
   Compiling substrait-explain v0.2.0 (/Users/wendell.smith/go/src/github.com/DataDog/substrait-explain)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.97s
     Running `target/debug/substrait-explain convert -i /Users/wendell.smith/data/where-42.substrait.json -t text`
=== Plan
Root["?column?"]
  Project[42]
    Filter[!{RexType} => ]
      Read[!{ReadRel} => ]
Formatting issues:
  Error formatting output: Unimplemented Error writing RexType: Subquery textification not implemented
  Error formatting output: Unimplemented Error writing ReadRel: Unexpected read type Some(VirtualTable(VirtualTable { values: [], expressions: [Struct { fields: [Expression { rex_type: Some(Literal(Literal { nullable: false, type_variation_reference: 0, literal_type: Some(I32(0)) })) }] }] }))
[ Process Exit Code: 1 ]

Note the errors and the non-zero exit code.

[wackywendell]

Needed for newer Rust clippy version

[wackywendell]

These have been disabled for a long time, no need to keep them

[gord02]

Removing the return and ; creates an expression, but is anything done with this expression now? Is there still error propagation?

[wackywendell]

Yes - an expression without a ; at the end of a function (or any {…} block) is the same as a return. This is considered more idiomatic in Rust; see this clippy lint.

Note also that this function returns a Result<…>; if you add a ; to this line without the Result, it would fail to compile because there would be nothing returned from this branch.

[gord02]

Is this a formatting issue or is this validation of the reference?

[wackywendell]

It's both - it's invalid as a plan, therefore can't be formatted. At formatting time, we use the function reference to look up the function name in the extensions, and then print the function name in the expression; if the function reference is valid, then the function name can't be printed.