Add manual error tracing

Error messages and function names/line numbers are traced via
__func__ or __FUNCTION__, __LINE__, etc (or just "unknown" if
not available). A manual stack is kept onto which these values
are pushed which can then be dumped to stderr/stdout.

stderr is used to report this information since no other
facility appears to be provided.

src/err.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2018 Diamond Light Source Ltd.
+ * Author: Charles Mita
+ */
+
+
+#ifndef NXS_XDS_ERR_H
+#define NXS_XDS_ERR_H
+
+
+#define ERR_MAX_FILENAME_LENGTH 64
+#define ERR_MAX_FUNCNAME_LENGTH 128
+#define ERR_MAX_MESSAGE_LENGTH 1024
+#define ERR_MAX_STACK_SIZE 128
+
+
+/* obtain __func__ from GCC if no C99 */
+#if __STDC_VERSION__ < 199901L
+# if __GNUC__ >= 2
+#  define __func__ __FUNCTION__
+# else
+#  define __func__ "<unknown>"
+# endif
+#endif
+
+#if __GNUC__ >= 2
+# define __line__ __LINE__
+#else
+# define __line__ 0
+#endif
+
+#if __GNUC__ >= 2
+# define __file__ __FILE__
+#else
+# define __file__ "unknown"
+#endif
+
+#define ERROR_JUMP(err, target, message) \
+{ \
+	push_error_stack(__file__, __func__, __line__, err, message); \
+	retval = err; \
+	goto target; \
+}
+
+void push_error_stack(const char *file, const char *func, int line, int err, const char *message);
+
+void dump_error_stack(FILE *out);
+
+void reset_error_stack();
+
+#endif /* NXS_XDS_ERR_H */