improve partialerror messages/labels

[swarnimarun]

All the below locations don't provide any meaningful labels for diagnostics produced by miette.

Is very easy to fix, eg https://github.com/prefix-dev/rattler-build/pull/398

❯ rg "_partialerror\!\((.*)\)" --glob=\!src/recipe/error.rs
src/variant_config.rs
54:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping,))
669:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping))

src/recipe/parser.rs
85:            .ok_or_else(|| _partialerror!(*root_node.span(), ErrorKind::ExpectedMapping,))?;

src/recipe/custom_yaml.rs
1072:        Url::parse(self.as_str()).map_err(|err| _partialerror!(*self.span(), ErrorKind::from(err)))

src/recipe/parser/test.rs
76:                Err(_partialerror!(*self.span(), ErrorKind::ExpectedMapping,))?
157:                Err(_partialerror!(*self.span(), ErrorKind::ExpectedMapping,))?

src/recipe/parser/package.rs
38:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping,))
110:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping,))
155:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedScalar))
163:            .map_err(|err| _partialerror!(*self.span(), ErrorKind::from(err),))

src/recipe/parser/build.rs
113:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping))
225:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping))
409:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedScalar,))?
435:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedScalar))
443:            .map_err(|err| _partialerror!(*self.span(), ErrorKind::EntryPointParsing(err),))

src/recipe/parser/about.rs
91:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedMapping,))
167:            .ok_or_else(|| _partialerror!(*self.span(), ErrorKind::ExpectedScalar,))
176:            .map_err(|err| _partialerror!(*self.span(), ErrorKind::from(err),))?;

src/recipe/parser/source.rs
340:                    let sha256_out = rattler_digest::parse_digest_from_hex::<Sha256>(sha256_str.as_str()).ok_or_else(|| _partialerror!(*sha256_str.span(), ErrorKind::InvalidSha256))?;
345:                    let md5_out = rattler_digest::parse_digest_from_hex::<Md5>(md5_str.as_str()).ok_or_else(|| _partialerror!(*md5_str.span(), ErrorKind::InvalidMd5))?;

src/recipe/parser/output.rs
155:                            _partialerror!(output_value_span, ErrorKind::ExpectedMapping,),
163:                            _partialerror!(*value.span(), ErrorKind::ExpectedMapping,),