Breakpoint.ensureMarker() not MT safe and should provide more info

[iloveeclipse]

• made Breakpoint.fMarker volatile so Breakpoint.ensureMarker() always see the last state
• changed error message to indicate what was the real problem - not set or not existing marker.

Fixes #1717

[iloveeclipse]

Error in case breakpoint is on missing resource:

!ENTRY org.eclipse.debug.core 4 5012 2025-01-29 17:14:33.929
!MESSAGE Breakpoint marker does not exist: Marker [on: /A/src/Issue3280.java, not found]

Error in case breakpoint marker is not set at all:

!ENTRY org.eclipse.debug.core 4 5012 2025-01-29 17:15:49.814
!MESSAGE Breakpoint does not have an associated marker.

[github-actions]

Test Results

 1 755 files  ±0   1 755 suites  ±0   1h 29m 21s ⏱️ - 1m 4s
 4 170 tests ±0   4 147 ✅ ±0   23 💤 ±0  0 ❌ ±0 
13 107 runs  ±0  12 940 ✅ ±0  167 💤 ±0  0 ❌ ±0 

Results for commit 70df70f. ± Comparison against base commit fbae6a1.