Strange syntax highlighting behavior with ALE on Neovim

## Information

NVIM v0.10.0
Build type: Release

Operating System: Arch Linux

## What went wrong

On May 16th I updated to the then-latest version of ALE and noticed that ALE had started using underlines for error/warning highlighting instead of my usual reverse color highlights. I thought this was odd but the colors were bright and visible so I ignored it. Later on May 25th Neovim updated to its new 0.10.0 version and broke my theme by setting `termguicolors` on by default. Once I fixed this by turning that setting back off I noticed ALE was still using underlines for/warning highlighting except now they were in pale washed out pinks and yellows so I decided to investigate.

I discovered that ALE seems to be ignoring my `ALEError` and `ALEWarning` highlights entirely. Instead it's using `DiagnosticUnderlineError` and `DiagnosticUnderlineWarn`. Using these two highlights I was able to customize the color of the underline (or change it from an underline back to a reverse highlight if I wanted). The symbols in the sign column are still styled correctly.

This behavior persists in the latest version of ALE.

I'm not sure if this is a bug or just me not understanding something but I wasn't able to find any documentation for ALE mentioning `DiagnosticUnderlineError`/`DiagnosticUnderlineWarn` and it seems to contradict the documentation for `ALEError`/`ALEWarn` so I decided to report it.

## Reproducing the bug

1. Use the latest version of ALE and use a linter to highlight an error.
2. Compare the style to those displayed for various highlights when running the `:hi` command without any arguments.
3. Using Neovim may be required. I haven't tested on other versions.

### :ALEInfo

Expand

```
let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = v:null
let g:ale_command_wrapper = ''
let g:ale_completion_delay = 100
let g:ale_completion_enabled = 1
let g:ale_completion_max_suggestions = 5
let g:ale_disable_lsp = 'auto'
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 0
let g:ale_fixers = {}
let g:ale_history_enabled = 1
let g:ale_info_default_mode = 'preview'
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 50
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {'c': ['clangd'], 'cpp': ['clangd']}
let g:ale_linters_explicit = 0
let g:ale_linters_ignore = {}
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = v:null
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_root = {}
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 1
let g:ale_sign_error = v:null
let g:ale_sign_info = v:null
let g:ale_sign_offset = v:null
let g:ale_sign_style_error = v:null
let g:ale_sign_style_warning = v:null
let g:ale_sign_warning = v:null
let g:ale_sign_highlight_linenrs = v:null
let g:ale_type_map = {}
let g:ale_use_neovim_diagnostics_api = 1
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 'disabled'
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
```