Handle and report memory allocation errors

[solardiz]

blists does check for errors from malloc and realloc, but it does so in its lower-level wrappers such as buffer_grow, msgs_grow, and concat. The higher-level and /indirect callers of those do not always check whether an error occurred (example: uses of concat in html_error_real) or when they do they might not make it clear to the sysadmin what the issue really is (example: msgs_grow failure in begin_inc_idx will result in non-incremental index update with no reason given as to why, which is especially unexpected to a sysadmin given that the underlying realloc can sometimes momentarily need almost 2x more memory than the requested new size).

We should review these cases and introduce reasonable error reporting.

[aabc]

I agree.