Improve error when config file is missing

[benalleng]

When no config file exists, the config library builds from defaults alone, which fails with a confusing "missing field" error for version-specific settings like ohttp_relays. Detect whether any config file was actually loaded and, if not, append a hint telling the user where to create one or to use CLI arguments instead.

pulled from DanGould#10

Closes #855

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[coveralls]

Coverage Report for CI Build 27221858423

Coverage decreased (-0.06%) to 85.143%

Details

• Coverage decreased (-0.06%) from the base build.
• Patch coverage: 16 uncovered changes across 1 file (9 of 25 lines covered, 36.0%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
payjoin-cli/src/app/config.rs	25	9	36.0%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-cli/src/app/config.rs	1	80.35%

---

Coverage Stats

Relevant Lines:	14727
Covered Lines:	12539
Line Coverage:	85.14%
Coverage Strength:	370.76 hits per line

---

💛 - Coveralls

[zealsham]

tAck

No config file found. Searched: /Users/Apple User/Library/Application Support/payjoin-cli/config.toml, config.toml (current directory). Create a config.toml or provide configuration via CLI arguments.

do you think making it " config.toml in (current directory)" is better

[benalleng]

Ehh I'm not too plussed about perfect grammar in cli reponses