Handle a missing -P/--preinclude file the same as an INCLUDE

[Rangi42]

• If dependencies are not being generated, then a missing preinclude file is a regular error, which continues parsing but prevents final assembly.
• With -MG (to exit normally at the first missing dependency), a missing preinclude file prints its dependency and immediately exits normally.
• With -MC (to continue after missing dependencies), a missing preinclude file prints its dependency and continues parsing.

[Rangi42]

-MG, before:

$ rgbasm -Weverything -Bcollapse -M - -o /dev/null @exits-after-missing-preinclude/a.flags exits-after-missing-preinclude/a.asm
/dev/null: exits-after-missing-preinclude/a.asm
/dev/null: nonexistent-pre.inc
error: Error reading pre-included file "nonexistent-pre.inc": No such file or directory
    at exits-after-missing-preinclude/a.asm(0)
/dev/null: never reaches here
Assembly aborted with 1 error!

-MG, after:

$ ../../rgbasm -Weverything -Bcollapse -M - -o /dev/null @exits-after-missing-preinclude/a.flags exits-after-missing-preinclude/a.asm
/dev/null: exits-after-missing-preinclude/a.asm
/dev/null: nonexistent-pre.inc

-MC, before:

$ rgbasm -Weverything -Bcollapse -M - -o /dev/null @continues-after-missing-preinclude/a.flags continues-after-missing-preinclude/a.asm
/dev/null: continues-after-missing-preinclude/a.asm
/dev/null: nonexistent-pre.inc
error: Error reading pre-included file "nonexistent-pre.inc": No such file or directory
    at continues-after-missing-preinclude/a.asm(0)
/dev/null: nonexistent1.inc
warning: still going! [-Wuser]
    at continues-after-missing-preinclude/a.asm(7)
/dev/null: nonexistent2.inc
warning: and going! [-Wuser]
    at continues-after-missing-preinclude/a.asm(9)
Assembly aborted with 1 error!

-MC, after:

$ ../../rgbasm -Weverything -Bcollapse -M - -o /dev/null @continues-after-missing-preinclude/a.flags continues-after-missing-preinclude/a.asm
/dev/null: continues-after-missing-preinclude/a.asm
/dev/null: nonexistent-pre.inc
/dev/null: nonexistent1.inc
warning: still going! [-Wuser]
    at continues-after-missing-preinclude/a.asm(7)
/dev/null: nonexistent2.inc
warning: and going! [-Wuser]
    at continues-after-missing-preinclude/a.asm(9)