sync, info: improve error message for cache update failure (#684)

Before this commit, running a `configlet sync` or `configlet info`
command when all these conditions were satisfied:

- The problem-specifications cache was present and valid
- and the --offline option was not passed
- but we failed to update the cache due to a network error

produced an unhelpful error:

    $ configlet sync
    Updating cached 'problem-specifications' data...
    fatal: unable to access 'https://github.com/exercism/problem-specifications/': Could not resolve host: github.com

    failed to fetch 'main' in problem-specifications directory: '/home/foo/.cache/exercism/configlet/problem-specifications'
    exec.nim(68)             execAndCheck
    Error: unhandled exception:  [OSError]

With this commit, configlet explains what the user can do next:

    $ configlet sync
    Updating cached 'problem-specifications' data...
    fatal: unable to access 'https://github.com/exercism/problem-specifications/': Could not resolve host: github.com

    failed to fetch 'main' in problem-specifications directory: '/home/foo/.cache/exercism/configlet/problem-specifications'
    Error: Unable to update the problem-specifications cache.

    You can either:

    - ensure that you have network connectivity, and run the same configlet command again
    - or add the '--offline' option to skip updating the cache

    The most recent commit in the problem-specifications cache is:

        commit 7012a73188698b2b7a2c340c1753d0198a38d8fa
        Author:     Katrina Owen <katrina.owen@gmail.com>
        CommitDate: Mon, 17 Oct 2022 16:36:08 +0200

            Change wording in tournament instructions (#2135)

Fixes: #683

src/cli.nim

@@ -304,7 +304,7 @@ proc showHelp(exitCode: range[0..255] = 0, prependDashedLine = false) =
   let f = if exitCode == 0: stdout else: stderr
   if prependDashedLine:
     let dashLen = helpUncompressed.firstLine().len
-    f.write(&"\n\n{'-'.repeat(dashLen)}\n\n")
+    f.write(&"\n{'-'.repeat(dashLen)}\n\n")
   f.writeLine helpUncompressed
   if f == stdout:
     f.flushFile()
@@ -323,14 +323,17 @@ let
   colorStdout* = shouldUseColor(stdout)
   colorStderr* = shouldUseColor(stderr)
 
-proc showError*(s: string) =
+proc showError*(s: string, writeHelp = true) =
   const errorPrefix = "Error: "
   if colorStderr:
     stderr.styledWrite(fgRed, errorPrefix)
   else:
     stderr.write(errorPrefix)
-  stderr.write(s)
-  showHelp(exitCode = 1, prependDashedLine = true)
+  stderr.writeLine(s)
+  if writeHelp:
+    showHelp(exitCode = 1, prependDashedLine = true)
+  else:
+    quit 1
 
 func formatOpt(kind: CmdLineKind, key: string, val = ""): string =
   ## Returns a string that describes an option, given its `kind`, `key` and

src/sync/probspecs.nim

@@ -158,9 +158,26 @@ proc validate(probSpecsDir: ProbSpecsDir, conf: Conf) =
 
       # `fetch` and `merge` separately, for better error messages.
       logNormal(&"Updating cached 'problem-specifications' data...")
-      discard gitCheck(0, ["fetch", "--quiet", remoteName, mainBranchName],
-                       &"failed to fetch '{mainBranchName}' in " &
-                       &"problem-specifications directory: '{probSpecsDir}'")
+      try:
+        discard gitCheck(0, ["fetch", "--quiet", remoteName, mainBranchName],
+                        &"failed to fetch '{mainBranchName}' in " &
+                        &"problem-specifications directory: '{probSpecsDir}'")
+      except OSError:
+        const msg = """
+          Unable to update the problem-specifications cache.
+
+          You can either:
+
+          - ensure that you have network connectivity, and run the same configlet command again
+          - or add the '--offline' option to skip updating the cache
+
+          The most recent commit in the problem-specifications cache is:
+
+          """.unindent()
+        const format = "commit %H%nAuthor:     %an <%ae>%nCommitDate: %cD%n%n    %s"
+        let mostRecentCommit = gitCheck(0, ["log", "-n1",
+                                            &"--format={format}"]).strip().indent(4)
+        showError(msg & mostRecentCommit, writeHelp = false)
 
       discard gitCheck(0, ["merge", "--ff-only", &"{remoteName}/{mainBranchName}"],
                        &"failed to merge '{mainBranchName}' in " &