Helpful error message when the user fails to use an array type in TOML

This applies to fields such as `web3-url` which are mapped to array
in TOML in a way that may surprise the user.

Author: zah

beacon_chain/nimbus_binary_common.nim

@@ -184,13 +184,30 @@ template makeBannerAndConfig*(clientId: string, ConfType: type): untyped =
 
   # TODO for some reason, copyrights are printed when doing `--help`
   {.push warning[ProveInit]: off.}
-  let config = ConfType.load(
-    version = version, # but a short version string makes more sense...
-    copyrightBanner = clientId,
-    secondarySources = proc (config: ConfType, sources: auto) =
-      if config.configFile.isSome:
-        sources.addConfigFile(Toml, config.configFile.get)
-  )
+  let config = try:
+    ConfType.load(
+      version = version, # but a short version string makes more sense...
+      copyrightBanner = clientId,
+      secondarySources = proc (config: ConfType, sources: auto) =
+        if config.configFile.isSome:
+          sources.addConfigFile(Toml, config.configFile.get)
+    )
+  except CatchableError as err:
+    # We need to log to stderr here, because logging hasn't been configured yet
+    stderr.write "Failure while loading the configuration:\n"
+    stderr.write err.msg
+    stderr.write "\n"
+
+    if err[] of ConfigurationError and
+       err.parent != nil and
+       err.parent[] of TomlFieldReadingError:
+      let fieldName = ((ref TomlFieldReadingError)(err.parent)).field
+      if fieldName in ["web3-url", "bootstrap-node",
+                       "direct-peer", "validator-monitor-pubkey"]:
+        stderr.write "Since the '" & fieldName & "' option is allowed to " &
+                     "have more than one value, please make sure to supply " &
+                     "a properly formatted TOML array\n"
+    quit 1
   {.pop.}
   config