Treat write errors in markdown doc generation

This code was already using io.Writer, but was completely ignoring write
errors.

The most worrying part is how GenMarkdownTreeCustom used an unnecessary
buffer to then dump all of its contents on a file, and instead of
returning an error on file creation/writing, it would just exit the
entire program.

doc/md_docs.md

@@ -39,13 +39,13 @@ This will write the markdown doc for ONLY "cmd" into the out, buffer.
 Both `GenMarkdown` and `GenMarkdownTree` have alternate versions with callbacks to get some control of the output:
 
 ```go
-func GenMarkdownTreeCustom(cmd *Command, dir string, filePrepender, linkHandler func(string) string) {
+func GenMarkdownTreeCustom(cmd *Command, dir string, filePrepender, linkHandler func(string) string) error {
     //...
 }
 ```
 
 ```go
-func GenMarkdownCustom(cmd *Command, out *bytes.Buffer, linkHandler func(string) string) {
+func GenMarkdownCustom(cmd *Command, out *bytes.Buffer, linkHandler func(string) string) error {
     //...
 }
 ```