Fix missing MTD error handling - add specific error
Summary:
Currently, upgrades that fail because of a blank /proc/mtd are being classed as `UNSAFE_TO_REBOOT` errors. This is because to classify as a `BAD_FLASH_CHIP` error, Flashy is grepping for the particular string *'Cannot access MTD device'* whereas the error on the BMC is returning:
```
root@rsw006-oob:~# cat /proc/mtd
dev: size erasesize name
root@rsw006-oob:~# fw_printenv bootargs
Cannot open /dev/mtd1: No such file or directory
```
Let's fix this by skipping the string grepping logic completely, as we already know that we cannot see MTD at this stage
Also added as separate error class, as missing MTD is a specific issue that we see quite frequently and good to classify it separately
Test Plan:
Unit tests
```
$ ./tools/flashy/scripts/run_unit_tests.sh
...
...
...
142/142 unit tests passed
```
---
Will handle exceptions in oobgrader and test in following diff
Reviewed By: doranand
Differential Revision: D60115096
fbshipit-source-id: 77e0cb6aa18fdb71e4c711d3951c7282aab23b6a
