Improved file-not-found handling

[johnkerl]

For some file formats, file-not-found errors were appearing as an ugly panic with stack trace.

Before:

$ mlr --tsv cat nonesuch
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select]:
github.com/johnkerl/miller/pkg/stream.Stream({0x1400010bbb0, 0x1, 0x1}, 0x140001ea540, {0x14000154f00, 0x2, 0x2}, {0x1010dc070?, 0x1400011e008}, 0x1)
... more lines ...

After:

$ mlr --tsv cat nonesuch
mlr: open nonesuch: no such file or directory.