Only try to record for files

[idbrii]

Plugins like dirvish and netrw let you "edit" directories to see what's in them. Using them triggers a panic in gtm, so don't do that.

panic: runtime error: invalid memory address or nil pointer dereference [signal 0xc0000005 code=0x0 addr=0x20 pc=0x64d57e]

Also output the result of calling gtm so users can see whether it's actually a path failure. Otherwise, gtm-vim's message may be confusing (it led me astray).