Errors
100-149 are I/O errors, 200-255 are Fatal errors according to the delphi help files.
The External Exception error code is given a name in Windows.pas. Search for e.g.
STATUS_BREAKPOINT
Run-Time errors below 100 (and a few unrelevant ones above) is DOS error codes, here's a table (Shamelessly ripped from Ralph Brown's Interrupt list)
As you can see, you get a sharing violation (kind of makes sense, eh?) when you try to modify your exe.
Values for DOS extended error code: 00h (0) no error 01h (1) function number invalid 02h (2) file not found 03h (3) path not found 04h (4) too many open files (no handles available) 05h (5) access denied 06h (6) invalid handle 07h (7) memory control block destroyed 08h (8) insufficient memory 09h (9) memory block address invalid 0Ah (10) environment invalid (usually >32K in length) 0Bh (11) format invalid 0Ch (12) access code invalid 0Dh (13) data invalid 0Eh (14) reserved 0Fh (15) invalid drive 10h (16) attempted to remove current directory 11h (17) not same device 12h (18) no more files ---DOS 3.0+ --- 13h (19) disk write-protected 14h (20) unknown unit 15h (21) drive not ready 16h (22) unknown command 17h (23) data error (CRC) 18h (24) bad request structure length 19h (25) seek error 1Ah (26) unknown media type (non-DOS disk) 1Bh (27) sector not found 1Ch (28) printer out of paper 1Dh (29) write fault 1Eh (30) read fault 1Fh (31) general failure 20h (32) sharing violation 21h (33) lock violation 22h (34) disk change invalid (ES:DI -> media ID structure)(see #0981) 23h (35) FCB unavailable 24h (36) sharing buffer overflow 25h (37) (DOS 4.0+) code page mismatch 26h (38) (DOS 4.0+) cannot complete file operation (out of input) 27h (39) (DOS 4.0+) insufficient disk space 28h-31h reserved 32h (50) network request not supported 33h (51) remote computer not listening 34h (52) duplicate name on network 35h (53) network name not found 36h (54) network busy 37h (55) network device no longer exists 38h (56) network BIOS command limit exceeded 39h (57) network adapter hardware error 3Ah (58) incorrect response from network 3Bh (59) unexpected network error 3Ch (60) incompatible remote adapter 3Dh (61) print queue full 3Eh (62) queue not full 3Fh (63) not enough space to print file 40h (64) network name was deleted 41h (65) network: Access denied 42h (66) network device type incorrect 43h (67) network name not found 44h (68) network name limit exceeded 45h (69) network BIOS session limit exceeded 46h (70) temporarily paused 47h (71) network request not accepted 48h (72) network print/disk redirection paused 49h (73) network software not installed (LANtastic) invalid network version 4Ah (74) unexpected adapter close (LANtastic) account expired 4Bh (75) (LANtastic) password expired 4Ch (76) (LANtastic) login attempt invalid at this time 4Dh (77) (LANtastic v3+) disk limit exceeded on network node 4Eh (78) (LANtastic v3+) not logged in to network node 4Fh (79) reserved 50h (80) file exists 51h (81) reserved 52h (82) cannot make directory 53h (83) fail on INT 24h 54h (84) (DOS 3.3+) too many redirections 55h (85) (DOS 3.3+) duplicate redirection 56h (86) (DOS 3.3+) invalid password 57h (87) (DOS 3.3+) invalid parameter 58h (88) (DOS 3.3+) network write fault 59h (89) (DOS 4.0+) function not supported on network 5Ah (90) (DOS 4.0+) required system component not installed 64h (100) (MSCDEX) unknown error 65h (101) (MSCDEX) not ready 66h (102) (MSCDEX) EMS memory no longer valid 67h (103) (MSCDEX) not High Sierra or ISO-9660 format 68h (104) (MSCDEX) door open B0h (176) (MS-DOS 7.0) volume is not locked B1h (177) (MS-DOS 7.0) volume is locked in drive B2h (178) (MS-DOS 7.0) volume is not removable B4h (180) (MS-DOS 7.0) lock count has been exceeded B5h (181) (MS-DOS 7.0) a valid eject request failed