print.c: provide more detailed error messages if retrieving a package fails

fetch_maperror() translates error codes returned by libfetch to our error
codes. Handle those in apk_error_str(), returning error messages which
advise the user of the most likely fix.

A custom error code, EAPKSTALEINDEX, has been added for cases where
retrieving a package fails due to a HTTP error 404 or similar.

[TimoT: add also EAPKBADURL, as well as organize a bit better where the
EAPKSTALEINDEX is generated]

src/apk_defines.h

@@ -32,6 +32,8 @@
 #define NULL 0L
 #endif
 
+#define EAPKSTALEINDEX 1024
+
 static inline void *ERR_PTR(long error) { return (void*) error; }
 static inline void *ERR_CAST(const void *ptr) { return (void*) ptr; }
 static inline int PTR_ERR(const void *ptr) { return (int)(long) ptr; }

src/print.c

@@ -133,7 +133,25 @@ const char *apk_error_str(int error)
 	case ENOMSG:
 		return "archive does not contain expected data";
 	case ENOPKG:
-		return "package not available";
+		return "could not find a repo which provides this package (check repositories file and run 'apk update')";
+	case ECONNABORTED:
+		return "network connection aborted";
+	case ECONNREFUSED:
+		return "could not connect to server (check repositories file)";
+	case ENETUNREACH:
+		return "network error (check Internet connection and firewall)";
+	case ENXIO:
+		return "DNS lookup error";
+	case EREMOTEIO:
+		return "error code returned by repo server (try 'apk update')";
+	case ETIMEDOUT:
+		return "operation timed out";
+	case EAGAIN:
+		return "temporary error (try again later)";
+	case EINVAL:
+		return "invalid URL (check your repositories file)";
+	case EAPKSTALEINDEX:
+		return "file not available from repo server (try 'apk update')";
 	default:
 		return strerror(error);
 	}