add some fail tips on nimbus_beacon_node deposits import

[xalexec]

show some fail tips on import invalid keys

project https://github.com/PufferFinance/coral Puffer,
can generate validators file,but the keystore file not ending by .json
I use nimbus_beacon_node deposits import but nothing is shown,at first I thought this was correct.
But Validator not working.
I check the code and submit pr to improve it.

[tersec]

needs to target unstable branch

[xalexec]

needs to target unstable branch

done.

[xalexec]

some update for this pr?what can I do

[tersec]

Main points:

• it adds a couple of flag variables to track state, which seems suboptimal, but I need to check closer if there's just no better way of handling this
• the error/log messages are have less-than-perfect grammar/spelling/etc
• not convinced of semantics of invalidFlag and/or message contents in case of multiple deposit directories, some of which are valid and some not

Will look more closely at these

[github-actions]

Unit Test Results

         9 files  ±  0    1 325 suites  +3   32m 41s ⏱️ + 3m 8s
  4 986 tests +  4    4 638 ✔️ +  4  348 💤 ±0  0 ❌ ±0 
20 814 runs   - 15  20 410 ✔️  - 15  404 💤 ±0  0 ❌ ±0 

Results for commit b642db9. ± Comparison against base commit 1b30dcc.

♻️ This comment has been updated with latest results.

[xalexec]

Reset the code,give up fatal exit

let importedFiles = walkDirRec(importedDir).toSeq
  if importedFiles.len == 0:
    fatal "No keystore file found at keystore path", key_path = importedDir
    quit 1
    
  for file in walkDirRec(importedDir):
    let filenameParts = splitFile(file)
    if toLowerAscii(filenameParts.ext) != ".json":
      warn "Keystore file must have a .json extension", file = file
      continue

How about this