Improve errExit error messages

Changes:

* Move msg to the end of errExit (right before perror(3p))
* Include the full file path (within the repository)
* Add "()" to function name for clarity

Before:

    Error malloc: main.c:123 main: Cannot allocate memory

After:

    Error src/firejail/main.c:123 main(): malloc: Cannot allocate memory

Note: This clarifies which is the exact file that the error message
comes from, as there are many source files with the same name.  For
example:

    $ git ls-files 'src/*/main.c' | wc -l
    20

1 files changed, 2 insertions, 1 deletions

diff --git a/src/include/common.h b/src/include/common.h
index 23b00b6f8..4a2b8c1bf 100644
--- a/src/include/common.h
+++ b/src/include/common.h
@@ -41,7 +41,8 @@
 
 #define errExit(msg) do { \
         char msgout[500]; \
-        snprintf(msgout, 500, "Error %s: %s:%d %s", msg, __FILE__, __LINE__, __func__); \
+        snprintf(msgout, 500, "Error %s/%s:%d %s(): %s", \
+                 MOD_DIR, __FILE__, __LINE__, __func__, msg); \
         perror(msgout); \
         exit(1); \
 } while (0)